Extending Your Home – The Ultimate Beginner’s Guide to Building a House Extension

What can you build under permitted development?

One key question you need to ask yourself when setting out on an extension project is whether you’ll be able to complete the addition under permitted development (PD), or whether you need formal planning permission.

Permitted development can allow you to build an extension – as long as it meets certain criteria and size limits – without the need to apply for formal planning permission. These are national, government-level guidelines as opposed to local authority guidelines, which differ considerably between each other.

Generally speaking, PD covers rear extensions, two-storey extensions, side extensions, loft and garage conversions, some front porch extensions. It also applies to things like changes to windows, cladding and render, size of patios and gardens etc. “Nine times out of 10, you want more from an extension than what PD will allow you to do,” says James Henderson from Fraher. “For very simple projects, PD can be great, as it’s faster and cheaper. However, for the more complex projects, it’s almost always going to require formal planning permission.”

The owners of this period property were keen to restore the vintage feel. Fraher designed a side return extension to resemble a Victorian glass house, flooding the space with natural light. Photo: Chris Wharton

Whether your house extension falls under PD or not, it’s important to refer to your local authority’s planning guidance to make sure you’re adhering to the rules that apply to your project and the area in which it’s located. For example, tighter restrictions apply to work being undertaken on houses in protected areas. Different rules also apply to listed properties, for which you’ll also need to secure listed building consent to make any alterations.

Permitted Development (PD) rules for extensions

As stated by the Planning Portal, this is what you can do under permitted development:

• You can build up to 8m out from the rear of a detached house in England, or 4m in Scotland and Wales.
• Only half the area of land around the existing house can be covered by an extension.
• Extensions cannot be higher than the highest part of the home’s existing roofline.
• Where the extension is within two metres of the plot’s boundary, the height at the eaves cannot exceed 3 metres.
• Extensions cannot be built forward of the home’s main elevation or where it fronts a highway.

Permitted development doesn’t include additions such as verandas, balconies, TV aerials, chimneys or alternations to the existing home’s roof. If you do build an extension under PD, they you may wish to obtain a Lawful Development Certificate, which proves that all the work you’ve done meets planning regulations. This is particularly helpful if you ever plan to sell your home down the line.

When do you need formal planning consent for an extension?

If you want a large addition or to make a design statement, you might find the parameters of permitted development fairly limiting. Any innovative details or standout cladding materials that differ to the existing property are likely to require formal consent.

If you decide to go down the formal planning route, the first step is to develop a set of drawings. Working with a design professional (keep reading for more) typically yields the best chance of success, as they’ll have the skills to create a scheme that realises your property’s full potential. If you need to carry out additional appraisals, such as flood risk assessments, ecological surveys etc, this is the time to appoint the relevant external consultants.

Konishi Gaffney Architects transformed this detached Victorian villa in Morningside, Edinburgh, into a light-filled, family-focused home. The project converted the former maid’s quarters into a double-height kitchen, dining and living space, with a mezzanine study above and extended eastwards to accommodate a generous kitchen. Photo: Nanne Springer

If your scheme is sensitive – for example, if you live in a listed property or a location with a specially designated status – going through the pre-application process could be beneficial. Though it can add a few weeks to your project schedule, it’s a chance to get early feedback on your house extension’s design and help identify any potential issues that may impede your scheme.

Once you have planning permission for your extension, it can be tempting to get building right away – but it’s important to check whether any pre-commencement conditions need to be discharged first. Ascertain if any documentation is required, such as a Party Wall Award, or a build-over agreement for development close to a mains sewer. You may also need Building Regs approval (keep reading to find out more). Finessing the construction details now can also save time and money on site.

How to get planning permission for an extension

When do you need a party wall agreement?

If you’re planning to work on walls that you share with neighbouring properties, or building close to an adjacent boundary, you’ll need to make sure you comply with the Party Wall Act. This legislation aims to prevent and resolve disputes. This means letting your neighbours know of your intention to extend, and how the work will be done. A party wall surveyor may need to resolve any disagreements and set parameters to protect neighbours’ property.

Building Regulations for extensions

Even if planning consent is not needed, the work must still comply with Building Regulations. You can either send your local council a full plan submission (best for high-value schemes), which the authority will then check against the current guidelines, or a building notice of your intent to start. Building notices tend to be for smaller projects (extensions, garages, loft conversions, etc). The work will be inspected at key stages.

For safety of the public and people using the building, Building Regulations will check the following factors:

• Foundations
• Damp-proofing
• Structural issues
• Insulation
• Ventilation
• Heating
• Sanitation
• Accessibility
• Fire protection and means of escape

How will your existing home impact your extension’s design?

Having outlined your objectives, it’s important to assess the design opportunities and constraints specific to your property. The goal is to create new spaces that seamlessly integrate with your existing home. Consider matching window depth, ceiling heights and roof angles to create continuity between old and new, ensuring materials and proportions complement the original architectural character.

Reconfiguring your existing spaces can enhance the flow of your whole home, without making them subservient to the new addition. Think about how key areas – such as the kitchen, living and dining rooms – can benefit from indoor-outdoor living, particularly in warmer seasons. Positioning high-activity zones away from bedrooms, studies and other quieter areas can further improve usability.

A unique blend of stacked white glazed bricks with green mortar characterises this wraparound house extension in Nunhead. Designed by Yard Architects, the project involved reconfiguring the house’s internal layout to create a new open-plan kitchen-diner, which makes use of a light, minimalist scheme and is connected to the garden via green-framed sliding doors. Photo: Richard Chivers

Orientation plays a significant role in determining how your extension interacts with natural light and seasonal temperature changes. “Orienting your home correctly can allow for passive heating in colder months but can also prevent overheating in the warmer months,” says Opinder Liddar, director at Lapd Architects.

Site access considerations might influence your choice of construction method. Properties in urban areas, for example, may have limited space for builders and materials on site, which might require careful coordination, phased deliveries, and off-site prefabricated construction methods. Similarly, homes with trees and animal habitats nearby may require specialist surveys to determine how close an extension can be built without disrupting the existing ecosystem. While self builders are exempt from Biodiversity Net Gain requirements, integrating features such as wildflower roofs or habitats can contribute to a more sustainable design.

Who can design your home extension?

Engaging a qualified architect is a great way to achieving a high-quality extension design, but there’s no legal obligation to use one. In fact, there are many professionals who can successfully draw your extension plans without the same level of qualification. The choice depends on what you want and what your budget is. So, who can design your extension? Popular house extension design routes include:

If you’ve already got a strong idea of what you want out of your extension, then draughtspeople can draw up technical plans for a builder to work from. “If you want a simple lean-to small side extension to add a couple of extra metres onto your kitchen, then potentially you could go to someone who can use computer-aided design (CAD) to put together drawings for you,” says Caroline. Bear in mind you may still need input from a structural engineer and other professionals.

Build It readers Harry and Sara Montatt undertook a complete internal overhaul of this 1975 three-storey London terrace and extended it to create a space ideal for growing their young family. They engaged Lumin Architects to help them create a light-filled rear extension which now houses an open-plan kitchen-diner. The whole renovation and extension project cost the couple £1,642 per m². Photo: Anna Stathaki

“Research your designer before you appoint them and make sure they offer the kind of service you want,” says John Payne, director of Blackrock Architecture. “Look at the projects they’ve done to see if they’re aligned with the scale and style of extension you’re keen to achieve.”

Which heating system should you specify for your extension?

Taking on an extension is a great opportunity to consider the energy efficiency of both your existing home and the new addition, maximising the benefits of your project. There are various factors to consider – from insulation levels and the new space’s heat distribution and emitters, through to ventilation strategies. These choices will be key to how enjoyable your new space is to use. So, what are your options for heating your extended home effectively?

Insulating the existing home

The first and most important step is to carefully consider the thermal performance of both your extension and existing house. “Insulation is critical – the better a property retains heat, the less energy is required to maintain comfortable room temperatures,” says Chris Hayes, UK technical manager at Roth UK. Upgrading the original home’s insulation means you can minimise extra heating loads brought on by the extension and get the best value out of your setup.

UK Building Regulations set minimum standards for extensions, which, while slightly less stringent than those for new builds, still demand significant attention to detail. The required U-values (which measure thermal performance, with lower values indicating better efficiency) are:

• Roofs: 0.15 W/m²K
• Walls and floors: 0.18 W/m²K
• Windows and doors: 1.2 W/m²K
• Rooflights: 2.2 W/m²K

Specifying sustainable technology

For most modern boilers that are in good condition, accommodating a modest extension may be feasible without significant upgrades. However, older or underpowered systems might need replacing to efficiently heat the expanded footprint.

If you’re undertaking a whole-house renovation (or retrofit) alongside the extension, you may consider switching to a heat pump. Under the government’s Boiler Upgrade Scheme (BUS), homeowners can access grants of up to £7,500 towards the installation of a heat pump. These systems are best suited to super thermally efficient, highly insulated properties, offering potential savings on energy bills and a reduction in carbon emissions.

Bear in mind that, as a heat pump runs at a lower flow temperature compared to a gas boiler, it relies on larger emitters such as oversized radiators or underfloor heating. So, a full system design is needed to ensure best results.

Choosing heat emitters – radiators & UFH

Water-based underfloor heating (UFH) systems are increasingly popular in extensions because they provide consistent warmth while freeing up wall space. UFH uses water heated to around 35°C, compared with radiators that require 60°C, often making it more cost-effective and efficient in open-plan spaces. It also works well with renewable heating systems such as heat pumps, helping you reduce energy consumption.

JK Floorheating systems are milled directly into your existing floor surface, reducing the need for additional insulation and therefore your retrofit underfloor heating costs down

“As soon as you select the designer for your extension, start talking about underfloor heating so they can plan it into the design of your build, says Chris. “Costly compromises will occur if you plan to include an UFH system too late.” Early planning avoids disruptive retrofits and ensures the system integrates seamlessly.

For extensions involving ground floor reconfigurations, a low-profile retrofit UFH system can deliver uniform heating while minimising floor build-up. Alternatively, for smaller projects, keeping existing radiators may be more practical. In bedrooms or bathrooms, where quick temperature changes are needed, traditional radiators or electric UFH systems can be effective.

“People often want to place the manifold in the corner of a house, but since every UFH pipe has to connect to it, choosing a more central location will result in a more efficient system,” says Chris. A central spot ensures even water distribution and consistent heating throughout, reducing cold spots and improving system performance.

Specifying a stove or fireplace for an extension

The benefits A stove can be a highly valuable addition to a home extension, offering both functional and aesthetic benefits. A stove can create a natural focal point while enhancing comfort and adding character to what is often a large, open-plan space. It can also boost energy efficiency by providing localised heating, reducing reliance on central heating and creating a cosy space year‑round.

Function When planning, it’s important to determine whether the stove will service as the main heat source, support an existing system such as a heat pump, or act primarily an atmospheric feature. This decision will guide choices around size, heat output, fuel type and the overall design of the space.

Stovax’s Studio 3 freestanding woodburner

Type of fuel Woodburning stoves offer traditional charm and high heat output, and make a fitting choice for those who love the ritual of a real fire. Gas stoves provide a similar living flame experience, but with instant control and consistent heat at the touch of a button. For maximum flexibility, electric fires offer the simplest installation and minimal maintenance.

Ventilation is a key consideration. Woodburning and gas stoves require appropriate flue systems and must comply with building regulations to ensure the safe removal of combustion gases. Adequate airflow is also essential – particularly in highly insulated extensions – to prevent negative pressure issues.

Location When choosing a location for the stove, aim to position it where it can be enjoyed from multiple areas. Ensure there is enough clearance from combustible materials, that is compliant with building regulations and consider how furniture layouts will interact with the heat source. If no chimney is present, a prefabricated flue system may be used for wood or gas stoves. Many gas models are also available as balanced‑flue versions, which do not require a chimney at all. Electric fires, meanwhile, need only a standard mains connection.

How to calculate your house extension budget

How much you should spend on your house extension will vary substantially from project to project. It all depends on what you’re trying to achieve with your extension, as well as the existing house. “It’s very rare that the work is just to the extension,” says Caroline. “This kind of project will usually involve some alterations to bring the existing house up to par, because when you’ve got a nice, shiny addition, the rest of the space might start to look a bit drab.”

When devising an extension budget and calculating how much your extension will cost, it’s important to be as clear as possible in the brief to your designer, in order to determine what building work is going to be involved. “Adjust the size of your extension to meet your budget rather than the other way round,” says Caroline.

Oliver Leech Architects have transformed this Victorian house, set within a conservation area in Herne Hill. The sensitive redesign involved replacing a network of small, disconnected rooms with a stepped extension containing a social, open-plan kitchen-living-dining area. The exterior comprises Con Mosso bricks, covered in a lime slurry to achieve a soft, stone-like textured finish. Photo: Jim Stephenson

The age of the property could also reveal further improvements that need to be made, such as electrics and plumbing. You may also choose to upgrade your main home’s insulation to meet modern living standards if you’re extending a period property. “Look to put your money into improving the fabric of your existing property. Any funds leftover can then go towards an extension,” adds Caroline.

How much does an extension cost to build?

Anyone looking to extend their home will question ‘how much does an extension cost?‘ and the result will vary significantly on a case-by-case basis. We have developed a series of benchmark costings for extensions, based on a main contractor build route. This is how much your house extension could cost per m²:

Single-storey extensions

In the UK, costs for a single-storey rear extension will typically sit between £1,800 and £3,000 per m². This could rise to around £2,500-£4,000 per m² in areas like London. These figures are based on creating a usable living space, but exclude the final decoration, fees, surveys and VAT. Here’s a typical cost breakdown for a mid-range, 30m² extension outside of London, assuming a total build cost of around £60,000.

Side-return extensions

Usually built as single-storey structures, side-returns fall within similar price bands – but the compact footprint may add a small uplift. From £2,300 to £2,900 per m² for a standard spec.

The large glazed pivot door in this side return extension by Bradley Van Der Straeten Architects is just one of the focal points bringing design flair to this home. Inside, a curved edge reaches up from the original ceiling to meet a large rooflight over the side addition, while playfully laid recycled bricks bring pattern and interest to the external elevation. Photo: French + Tye

Wraparound extensions

Construction costs per square metre are broadly similar to those for other types of single-storey extension, generally starting from around £2,100 to £2,700 per m², depending on specification.

Two-storey extensions

Because the same foundations and roof structure serve two levels, two-storey extensions can offer better value per square metre of space added, with prices typically starting at around £1,800 to £2,400 per m², rising to £2,600+ per m² for higher-quality finishes.

Oakwrights masterminded this two-storey extension, which holds a kitchen-dining-living area and master bedroom suite. The extension features a striking glazed gable and two sets of bifold doors that make for a fabulous, light-filled interior. The project cost a total of £195,000. Photo: Richard Kiely

Basements

Works to convert an existing basement might start from around £1,000 per m², while a full new basement extension typically begins at around £3,000 per m². These indicative figures are to waterproofed shell and floor screed only (so you’ll need to add fit-out costs on top). So, it’s a relatively expensive way to create new space – but in the right location, you’ll more than recoup the cost in added value.

Loft conversion

A straightforward rooflight conversion might start from around £1,800 per m²; with dormer loft conversions beginning at £2,000 per m². More complex alterations such as hip-to-gable or mansard extensions start at approximately £2,550 per m².

Fraher & Findlay architects were approached by a young client who was looking to transform their first floor flat and add more space at roof level. They created a scheme with the kitchen and dining spaces looking out over the rooftops with the bedrooms positioned underneath. The loft extension added 27m² of new living space and was completed on a budget of £260,000 (for the whole-flat renovation and loft extension). Photo: Chris Wharton

YOUR COMPLETE GUIDE TO HOUSE EXTENSION COSTS

Who should project manage your extension project?

The building and project management route you choose for your extension project will impact costs. An average single-storey rear extension (outside of London) could cost around £2,400 per m² built using a main contractor. The different options could help you save on this overall cost.

Main contractor – standard option

The figures above in our extension costs section are based on a main contractor route, where a general building firm manages the project to completion on your behalf, using a standard contract. You can bring prices down by taking on more of the responsibility yourself. Below you can see typical indicative savings for other popular build routes.

Builder plus subcontractors – potential saving 10%

You could potentially reduce build costs by circa 10% by hiring a main contractor to complete the structure to watertight stage. At this point you take over from the main contractor as a project manager and the remaining work is undertaken by subcontractors (individual trades), whom you manage through to project completion.

Self project managed – potential saving 20%

By project managing the entire extension yourself, including the main structural phase, you could knock up to 20% off total build costs. This route doesn’t involve undertaking any construction works yourself, but rather fully managing the subcontractors on a DIY basis. So you are both client and building contractor, hiring trades and supplying plant, machinery, tools and most of the materials.

DIY extension – potential saving 25%

Many will ask themselves ‘can I build my own extension?’ and the answer is yes. Undertaking a large proportion of the build on a DIY basis could enable you to reduce project costs by as much as 25%. This route assumes you’ll use trades for the key structural and infrastructure works, but will carry out much of the second fix tasks, landscaping, general labouring, decorating, tiling etc yourself. You will also be project manager, buying most of the materials and supplying all tools, plant, scaffold etc.

Who should build your house extension? 

Many design professionals will have a network of trusted contractors and can put your project out to tender. “We always try to matchmake the builders we work with to our clients and their projects,” says James. “We don’t do an open or competitive tender. Instead, we introduce the contractor to the homeowner and negotiate on their behalf to find the right price for the project.”

In many cases your designer may also offer contract administration to oversee the construction phase, ensuring the extension is built according to design and to a high-quality standard. Alternatively, you might ask them to provide ad hoc advice during the work and check in at the end of the project to help identify any snagging issues.

Alice Constable Maxwell‘s home has been upgraded with a dormer loft conversion and a light-filled rear kitchen-diner extension with colourful glazed features and a charming interior scheme. The project came in at £1,916 per m² for the full renovation and extension works. Photo: Simon Maxwell

The key is to know exactly what you’re getting as part of their service. “Some designers will only offer drawings, but some, such as ourselves, stay with the client throughout the project from beginning to end,” says John at Blackrock Architecture.

If you opt for a designer to produce solely the technical plans for your extension, you’ll need to do your own thorough research to find a good builder. “Look up their firm on Companies House – have they got cash in the bank or are they likely to go bust?” says James.

Word of mouth is often an effective way to find someone reliable, particularly from neighbours, as the builder will have experience with your type of house and area. “Get a contractor who’s worked on a similar type of project before, as they’ll know what to expect and be more accurate with their costings,” says Caroline.

Do you need home extension insurance

Many homeowners mistakenly believe that their builder’s insurance or standard home insurance policy will cover the risks of a house extension. Unfortunately, this is often not the case. “These policies typically won’t offer the right kind of cover for an extension,” says Harry Connor, head of sales at BuildStore Insurance Services. “It’s important to have the correct protection in place to safeguard yourself and your property – plus the construction works and on site tools or machinery.”

Standard home insurance policies are typically geared towards protecting a finished home and may not extend coverage to new works or unoccupied properties. Even if your provider agrees to add on extension cover, many elements such as theft of materials or equipment, or damage caused during construction, is likely be excluded. A comprehensive extension insurance policy is the best way to protect your investment, yourself and your home if unexpected issues arise during your project.

What’s covered in a home extension insurance policy?

A specialised house extension insurance policy is specifically designed to cover the unique risks you will face when carrying out a project.

• The Existing property and the extension work will be covered up to the reinstatement value stated in your policy. Employers’ and public liability insurance is included to protect anyone working on site, including family and friends, and their property. It covers third-party injuries or property damage.
• Cover for unoccupied homes isn’t usually included under your standard home insurance, but a tailored extension policy provides protection both before and during the construction phase.
• Legal expenses should be covered and your provider may also offer advice and support in the case of disputes – helping you to avoid costly legal battles.

How much does extension insurance cost?

The cost of a house extension insurance policy varies depending on several factors, including the size of the project, location (ie mining area, flood zones), whether your existing building is listed or in a conservation area and if you need party wall agreements.

“A typical 12-month policy might cost between £750 and £2,000,” says Harry from BuildStore. For smaller projects, six months of coverage may be more suitable. “On a £200,000 house with an extension budget of £100,000, you might pay around £480 for six months of cover,” says Simon Middleton, managing director at Protek Group. You should consider this cost as part of your overall project budget.

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Do you need structural warranty for a home extension?

While it’s not a legal requirement, a structural warranty is a valuable addition to a house extension project – offering long term protection against potential construction defects. This kind of cover can be particularly useful for large extensions, providing added peace of mind for sizeable investments. Issues with the foundations, walls, roofing, or other structural elements that could compromise the integrity of the extension would be covered.

“As the developer of the works, you remain liable under the Defective Premises Act for those works and a structural warranty helps risk manage that exposure,” says Simon from Protek Group. It can also be important if you plan to finance the project or sell the property in the future, as lenders often require this type of coverage before approving loans. Additionally, warranty providers will inspect the works before providing cover to ensure they are being built to standard, which offers another layer of assurance.