Who Should Design My House Extension?

Whether you’re planning a kitchen-diner that connects with the garden or a master bedroom in the loft, make sure the layout and look of the new addition suits your needs and lifestyle. Here’s how to get the design right.
by Lucy Aplin
6th October 2020

Adding space to your property is a great way to improve how your home works for you, plus extending is likely to increase its value. Whatever the reason behind your build, someone needs to put together plans for how the new part of your home is going to take shape. Here are the key options when it comes to who can design your extension.

Whatever route you go for, always select someone with professional indemnity insurance, and get your own specific extension cover to protect your project.

OPTION 1: Architect

When people hear home design, they typically think of an architect. These professionals need to complete seven years of training and have to be accepted by the Architect’s Registration Board (ARB).

While anyone can call themselves a designer, you need this training and professional status to be legally known as an architect. However, this does mean that their services come with a higher price tag than some other design routes.

Not every home extension project requires the skills of an architect, but there are certainly situations where it’s a good idea to have them on-board. These professionals often have specialist knowledge that makes them particularly useful for more complex, interesting schemes.

For instance, if you want to extend a period property, an architect with experience in heritage homes will help you develop a sensitive design that will gain planning permission. That’s true on complicated builds, too.

“Employ an architect for a project that involves a number of professionals – a basement extension would need a structural design, flood risk assessment, soil tests, and much more. An architect will have the broad experience needed to pull all of this together,” says Peter Morris from Peter Morris Architects.

Remember that not every registered architect is right for every project – make sure you pick one that has experience in the kind of style and build you’re working on. Look at their work to see if they’ve done similar projects to what you want.

Glazed extension with floating corners

This extension was designed by Peter Morris Architects. The clients wanted a home with plenty of natural light that they could enjoy with their family, which is achieved with the glazed doors and floating corner

“Ask for at least three references from previous clients, find out if the architect knows any suitable builders and make sure they have enough time and resources to do your extension,” suggests Peter.

You can access the full list of registered architects, but remember to shop around. “Don’t always just go for the cheapest quote,” says Peter. “Look to employ someone who seems positive, proactive and flexible.”

OPTION 2: Designer

These are a more affordable alternative to architects, yet they can still offer the skills needed to create striking home extensions.

Architectural technologists have a professional body, the Chartered Institute of Architectural Technologists (CIAT), which works in a similar way to the ARB. MCIAT members will have undergone professional training, meaning they have the credentials to make a success of your project.

While being a chartered architectural technologist is a protected title that requires registration with CIAT, someone doesn’t have to be associated with the body to call themselves an architectural technologist or the similar-sounding ‘architectural technician’.

Timber clad extension under permitted development

Most of the work on this extension by Architecture for London was completed under PD rights

A design and build package is another great option. Having a single point of contact can mean less hassle and you’ll benefit from the cost certainty of agreeing a fixed price up-front. Some builders offer an in-house design service and pre-fabricated extensions created off-site and craned into place tend to work this way, too.

Planning permission for extensions

Some extensions need formal planning permission and others do not. Your case will depend on a few factors, including if there are any restrictions in your area (such as areas of outstanding natural beauty), your project (for instance, listed buildings) and if work has be done previously. Even if it looks like your project falls under permitted development, it’s worth checking with the council before starting work.

Learn more: Beginner’s Guide to Planning Permission 

Permitted development

Every kind of extension needs some form of planning permission, whether you are designing a basement, porch, or double storey addition. Thankfully, many projects already have consent under what’s known as permitted development (PD). There are limitations to what can be built, but you could build an 8m-deep single-storey rear extension on a detached house in England under these rights.

However, you still have to notify your local council before you start work. You can check if your home benefits from permitted development by applying for a lawful development certificate (LDC) with your local council.

What if I need to apply for permission?

If your plans do not comply with PD then you will have to submit a formal planning application. Policies tend to be more restrictive for rural plots, and there are specific requirements for extensions built in conservation areas. If you are extending a listed building, then expect strict planning restrictions; an architect can help you to overcome these.

Often a basement conversion will not need formal consent; however large-scale, disruptive excavation projects will.

Usually, an extension must work architecturally with the property and surroundings. Avoid designing an addition that will block light or outlook for neighbours and show them your plans before submitting.

The simplest way to make a planning application is via the online Planning Portal. You can also download planning application forms from your local council website and send your forms via the post.

The design and build route is also common for loft conversions and basement projects. While this project option might be quicker, it’s not necessarily cheaper than using an architect or independent designer, so be clear about what you’re getting.

As with any service or product that you buy for your home project, it’s essential you do your research before purchasing. Check that your chosen designer has successfully completed similar projects to yours, ask for references or visit the homes they’ve worked on in person.


Many people choose to forego the professionals entirely and take on the challenge of designing their extension themselves. This is cheaper in the short term, but any minor mistakes could end up being costly to solve later on. For example, if you integrate certain modern materials in heritage schemes, you can cause damp issues in the future.

If you are considering going down the DIY approach, first identify your needs and consider how they can be met with the space available. Properly analyse the plot and investigate where the natural light is coming from. Consider the neighbouring vernacular styles and materials.

Recognise if you need formal planning permission (see below box for more on this).
Your builder’s quote will be based on your drawings and specification, so these need to be detailed if you want an accurate price.

Design software such as Build It 3D Home Designer or SketchUp is simple
to use and can help you visualise your plans. And if you’re looking for ideas, architect and Build It expert Julian Owen has plenty more design advice – check out Julian’s article hub here.

Main Image: Peter Morris Architects

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