Loft Conversion Complete Guide: Costs, Planning, Design & More

A loft conversion will add space and value to your home – but what necessary steps do you need to take to ensure a successful project? These are the top 10 questions to ask yourself for a loft conversion that meets your budget, design preferences and Building Regulations
by Paul Hymers
16th May 2024

Are you wondering whether you can or should upgrade your home with a loft conversion? Loft conversions and extensions are a great option for those seeking out additional bedrooms or living space without extending into the garden. But before you begin drawing up your loft conversion plans, it’s worth finding out exactly what it will involve and how it will affect your home’s current design, layout and overall functionality.

This process will involve assessing the roof and floor structure, the staircase design that’ll connect the two stories, how you’re able to get light into the loft conversion space and any insulation that’s required to ensure a habitable, cosy space.

From the costs to Building Regs and windows, this loft conversion step-by-step guide takes an in-depth look at the 10 key things you need to consider when weighing up whether a loft conversion is the right way to add more space to your property.

1. Can my home take the weight of a loft conversion?

Clearly building a loft conversion will add weight to your house and, although it may only be a modest increase, you’ll need to make sure that the structure of the building can take it.

To do this, you or your builder will need to expose the foundations and check them, together with any beams or lintels that will be asked to carry more weight.

exterior of bungalow renovation

With the help of Chambers McMillan architects, John and Susan Yates completely transformed this traditional single-storey build by converting the loft into livable space as they felt they could benefit more from the solar gain in a living zone instead of a bedroom. Some locals were concerned that the loft conversion’s dormer window would overlook them, but the model demonstrated this wouldn’t be the case. Photo: David Barbour

Your Building Control officer will also want to check all these elements, so dig a small hole to expose the foundations first. If it turns out that your house needs underpinning to support the extra weight of a loft conversion, it could double your budget before you start. So this is a key consideration when planning how to convert your loft.

2. How much head height does a loft conversion require?

Get your designer to illustrate clearly how much headroom you’ll have in your loft once it’s converted – people are often disappointed by how much space they have to actually stand up in and on plans this isn’t always clear.

Don’t forget you’ll have to accommodate a staircase leading up into the loft conversion space. To make the best use of space the new staircase should rise above the old one and not from within an existing bedroom. There’s not much point in converting the loft space if it means losing an entire room on the first floor.

Without the roof space for water tanks and plumbing, the heating and hot water system may have to be replaced with a sealed system. It’s better to have an unvented hot water cylinder than a combination boiler, but it will take up a cupboard-size room and you’ll need to find somewhere to put it.

amazing loft conversion ideas

EXPERT VIEW Calling in quotes for loft conversions

One of the key steps in the search for the right builder is understanding and comparing quotes. Deepak Sing Udassi from City Lofts London reveals why obtaining quality quotes is all about the planning.

Start by writing a design brief. Think about why you are undertaking this project and detail the must-haves, should-haves and could-haves. Then invest in quality drawings. For simple, non-structural home improvements you could probably go straight to the builder; for anything else hire an architect to produce detailed plans.

The next step is to shortlist prospective builders. Seek recommendations from friends and family, and find accredited professionals through the Federation of Master BuildersFind a Builder service. Invite a minimum of three builders to quote. Give the same detailed plans to each builder, and ask them the same questions, so you can easily compare quotes.

Check each quote you receive contains plans referencing everything in the scope (as well as things that are considered out of scope) and a full fixed price (stating whether it includes VAT). It should also detail any assumptions or special considerations the builder has made about the project.

Before you accept, carry out your own due diligence. List any concerns and have your builder reply in writing. Request references, visit past or current projects, and check their credentials (such as FMB registration) and insurance documentation.

The quotes you receive will only be as good as the pre-work you put in, the thoroughness of the plans you provide and the suitability of your builder shortlist.

Deepak Singh Udassi is MD of City Lofts London, a Master Builder company that specialises in high-quality loft conversions. He also runs RIBA architectural practice Architecture 100, specialising in house extensions. This 360-degree experience gives Deepak a valuable, end-to-end overview of the house extension process.

3. Building Regulations & Party Walls for loft conversions

Loft conversions always need approval under Building Regulations (irrespective of whether they need planning permission) so it pays to adopt the full plans application approach and have a detailed scheme approved before you find a builder.s

Having an approved design will take much of the risk out of the work and also mean the builder has a chance to give you a fixed quotation, rather than a vague estimate.

If your house is semi-detached or terraced don’t forget to notify your neighbour of your proposals, which will usually fall under the Party Wall Act 1996.

Your Building Control officer will inspect the work at various stages and on a final inspection should issue you with a completion certificate. Don’t settle any final accounts with contractors until you’ve received the certificate.

4. How do I alter my roof’s structure & floor joists for a loft conversion?

Most roofs are constructed with internal support struts in the loft, propping up the rafters and purlins (horizontal roof beams) in traditional cut and pitched roofs, and making up the web of braces in modern trussed rafter roofs.

All these have to be removed to make way for the new room and replaced with new supports that don’t encroach on the space available in the loft void.

There are many ways of altering roof structures for a loft conversion, but they all have one common element – the ceiling joists will almost certainly be inadequate as floor joists. This means that new floor joists are fitted alongside them, slightly raised above the ceiling plasterboard to avoid contact with it.

Loft Conversion Complete Guide: Costs, Planning, Design & More

Mike Tuck Studio have transformed this typical four-bedroom Victorian terraced property with a rear extension and dormer loft conversion. The new additions feature a distinctive, deep rust-coloured corrugated cement board facade, making a contemporary statement set against the traditional period home. Photo: Luca Piffaretti

These joists (often 200mm or 225mm in depth) will rise above the tops of the current ceiling joists to form the floor structure. Depending on their span they will bear either directly on to the existing wall plates of external and internal load-bearing walls, or on to newly installed beams.

In smaller lofts, it is often the case that the floor joists themselves will be used to support the sloping rafters. This is possible by constructing a dwarf timber stud wall 1m to 1.5m high, known as an ashlering, between the two. With the supporting ashlering in place, the internal struts and braces can now safely be removed.

EXPERT VIEW Loft conversion design considerations   

Mike Tuck from Mike Tuck Studio explains the different considerations to bear in mind when designing a loft conversion

What do I need to consider before deciding whether a loft conversion is right for me?

First up, you might want to consider if you have enough space in your loft. You may have enough space to stand in your loft but once the floor has been reinforced and the roof insulated and strengthened you could easily find it significantly smaller.

You will need to check if you live in a conservation area, listed building, or other area with planning restrictions on lofts, because this could restrict development. If in doubt, ask the local planning authority (LPA). You may be able to extend your roof to create a loft conversion under permitted development rights, meaning you won’t require formal planning permission, but beware there are technical restrictions on the size of the extension, materials used for external finishes and other details of the design. If the loft space is above an apartment, you should consider if you have the right in your ownership documents to develop it into a habitable space.

Do I need an architect for a loft conversion?

You don’t need an architect for a loft conversion and there are plenty of very competent loft conversion companies who will handle the process of designing and building the new space for you, however they tend to work according to a generic design for your type of property. An architect can create a specific design that takes into account local planning constraints and thinks carefully about common problems such as solar gain and overheating, and appropriate use of materials.

Lofts are usually very complex spaces, so having an architect involved in the design process can help you understand and visualise how it will all come together and how best the space can be maximised before any of the work takes place.

How will I connect my loft conversion to the storey below?

A lot of homeowners don’t realise that if they add an additional storey to a two-storey home, they will usually be required to form what is known as a protected means of escape. This means that all habitable rooms leading on to the stairs will require a certified fire door. If you already have an open-plan floorplan on your ground floor, this can cause a problem and needs to be thought about early on in the design process.

If you need to sacrifice a room for a staircase up to the newly-converted loft, you might want to question if the works are actually worthwhile. Also, the stair generally needs to arrive at the taller part of the new room because you need at least two metres of headroom above each step of the stair to comply with Building Regulations. There are other rules on how steep your stair can be that your builder will need to be aware of.

5. How can I design a staircase for my loft conversion?

Staircases are invariably tricky to design for loft conversions, as space for them is tight. Narrow winding flights are acceptable, but may prove impractical, because it’s difficult to get furniture up them.

Purpose-built staircase designs are around 10 times the cost of standard (off-the-shelf) designs, so bear this in mind when you’re planning your loft conversion.

If you do need a purpose-built loft conversion staircase, it pays to have the design approved by your Building Control officer before you actually commission them. Ask your joiner or builder to send Building Control a copy of the design.

As part of the fire safety upgrade for your loft (keep reading) your stairways should lead to a hall and an external door. If you have an open-plan layout where the stairs rise from a room, it is likely you’ll have to alter it, fitting a new partition wall or choice of escape routes.

How much does a loft conversion cost in the UK?

The Build It Estimating Service has developed a set of benchmark costings for loft conversions, based on a main contractor build route. This is how much your loft conversion could cost:

Available head height is a key factor when it comes to establishing the likely costs of creating habitable loft space. These figures are based on building a straightforward loft conversion that creates 40m² of internal floor area. The costs assume that the roof tiles do not need replacing, and are based on a main contractor or loft specialist route.

In general, prices for fully-finished rooflight loft conversions start from around £1,800 per m². A big factor here will be the number of roof windows inserted: these can cost around £1,200 each.

A typical dormer loft conversion will start at around £2,000 per m², fully-finished and ready to inhabit. If you’re extending the loft to add more space, such as with a hip-to-gable or mansard, expect to pay significantly more – budget from around £2,550 per m².

6. Where should I fit windows for a light-filled loft conversion?

You don’t need to make a lot of structural alterations to accommodate rooflight or skylight windows in your new loft conversion, which makes them relatively easy to fit. Typically the rafters on either side of the rooflight are doubled-up and trimmed across the top of the opening.

A popular alternative is to fit dormer windows, which are structures in themselves, as they have walls and a roof as well as the window itself.

At the rear of many homes dormer windows can fall into the permitted development quota and so may not require planning permission. At the front of the house, however, they will require planning permission, which is why you often see rooflights or skylights instead.

Dormer windows may be essential to maximise the headroom in the loft and provide useable space, but will need to be supported at the apex point (ridge). A ridge beam is installed beneath the apex before the dormer roof joists can themselves be fixed in place and the roof weathered.

It is at this stage, when the dormer windows are being constructed, that your loft conversion will be exposed to the elements, so you’ll need good temporary sheeting to protect against the weather.

CASE STUDY Colourful loft extension

Paul Archer Design are behind this contemporary loft conversion and extension to an Edwardian flat in North London. The former loft area, located on the flat’s second floor, has been completely upgraded to create a new living space with a pitched roof and large dormer windows on either side, accompanied by large windows to maximise natural light.

Bright & Colourful Loft Extension Project 

On the second floor, two further bedrooms and a main bathroom complete the space. The palette of materials has been carefully considered, with plywood for the bespoke built-in storage and terrazzo in the main bathroom.

Bright & Colourful Loft Extension Project 

The home features bright whites and wood details throughout, with a bright yellow kitchen island to bring a pop of colour to the top room. The project cost a total of £2,080 per m².

Photo: Andy Stagg

7. How can I ensure my loft conversion is fire safe?

Loft conversions on bungalows have little effect on the fire safety of your home, beyond making sure that the new windows are large enough to escape out of. But in a house where two storeys become three, there are implications.

The new floor will need at least 30 minutes of fire-protection, which could mean re-plastering the ceilings below it and the loft room will have to be separated by a fire door, either at the top or bottom of the new stairs. You’ll also need one escape-sized window per room – some skylight windows are made specifically with this in mind.

Contemporary Mansard roof conversion

Life Size Architecture, a Brighton-based team of architectural designers and technologists, designed this stunning rear and side extension and loft conversion to this home located in a conservation area. The architects commissioned Attic Conversions Ltd to carry out the works of the mansard roof addition, which features bifold door openings and a metal balustrade.

Self-closing door devices are no longer required in homes. They’ve proven to be a risk to children’s safety because they can trap tiny fingers. Instead, existing doors on the stairway (ground and first floor) should be replaced with fire-resistant doors or upgraded – and this should be indicated on your loft conversion drawings for Building Control.

As part of the electrical installation for a loft conversion, mains-powered smoke alarms should be installed on each floor of your home and these should be interlinked so that they all sound when one is activated. Most have a re-chargeable battery as a back-up that allows the supply to be extended from a lighting circuit if need be.

8. What kind of insulation does a loft conversion require?

With energy efficiency standards being increased, loft conversion insulation is more difficult to install than it once was. If you are replacing the roof tiles at the same time, you can insulate between the covering and the rafters, which will also achieve good airtightness.

Bright lounge in renovated 1930s home

Adrian and Maria Pearcey converted their 1930s house with a loft conversion to make space for an extra bedroom. For the new master bedroom in the attic, the couple wanted to maintain a sense of being up in the eaves of the dwelling. The zone feels bright and spacious with four large Velux windows allowing copious amounts of light to stream into the bedroom

If you’re not replacing the roof, the sloping ceiling will need insulation cut and fitted between the rafters, as well as on the underside of the rafters. As the plasterboard will have to be fixed to the rafters through the bottom layer of insulation, you will want this insulation to be as thin as possible.

You should use some high performance insulation (typically a foam board) for all of these areas. The ashlaring walls and dormer window structures will also need insulating with similar products before they are plasterboarded.

9. What level of sound insulation will my loft conversion require?

The new floor also needs soundproofing, and this is easily achieved by laying a mineral fibre quilt between the joists. Use the heavier, denser sound insulation quilt and not the lighter thermal insulation material, which is of no help here. The same goes for any internal stud partitions between bedrooms or bathrooms.

You should also consider insulating any party walls, both against heat loss and noise. A lining framework of timber stud will allow you to achieve both and you can cover it with sound-rated plasterboard.

10. How can I maximise storage in my loft conversion?

When you convert your loft you are, of course, going to lose storage space. Make the most of what you have by using the eaves behind the ashlaring – fit access hatches and have roll-out storage bins made to fit. And if you insulate down the rafter line to the eaves you’ll create a warm store for your belongings.

Zinc-Clad Loft Conversion Idea

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 areas looking out over the rooftops with the bedrooms positioned underneath. The loft conversion’s exterior has been clad in red zinc with a sedum roof on top to bring a diverse habitat to the building. The project was completed on a budget of £260,000. Photo: Chris Wharton

Built-in wardrobes are also a great feature in loft bedrooms, where standard units won’t fit – and are among the most innovative loft storage conversion ideas.

This article was originally published in 2020 and has been updated in May 2024.

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