Where to start with a home extension: advice from a builder and an architect

Finding really good tradespeople can be tricky, but it's worth putting in the groundwork to make the experience of building an extension as smooth as possible.

We've spoken to a builder and an architect to better understand what's involved when it comes to planning a new extension and what key questions you should be thinking about and asking before you instruct anyone and begin a project.

Skip to the bottom of the guide to also find out more about structural surveyors, architectural technologists and party wall surveyors.

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How to find an architect or builder for an extension

The best place to start is to try and find someone who has been through the process before and get their recommendations for good local architects, structural engineers and builders.

Which? Trusted Traders also has a great library of vetted and reliable traders local to you. 

To make your hunt a little easier, we've asked an architect and a builder what the most important questions are that you should be asking and what to look out for.

Top tips from a builder

Joseph Cole, director of All Build Developments, which is endorsed by Which? Trusted Traders, has shared his best advice for picking a builder.

His tips include preparing a spec, gathering quotes, checking insurance details, how long you might need to wait for a good builder to start a job, whether our not you should pay with cash, and dealing with the inevitable disruption that building work entails.

Which? members can log in now to unlock Joseph's insider insight.

If you're not yet a member, join Which? to get instant access to his advice.

Do I need an architect for an extension?

This is one of the most commonly asked questions when it comes to home extensions and renovations.

You don't need to hire an architect for an extension or home renovation project, but you'd receive more support throughout the build and probably a more considered finish that makes the most of the space you have. 

Architects are highly qualified in areas of law, contracts and project management. Many are members of the Royal Institute of British Architects (RIBA), who are legally obliged to carry professional indemnity insurance for the benefit of you and your project. Plus, they can specialise in areas such as interior or environmental design. 

Architects tend to be problem solvers, so hiring one gives you the opportunity to discuss ideas in detail and ask for advice, as well as benefit from their experience of working on similar projects. Some people find it easier to have a professional draw up the designs, lead and manage projects from inception to completion, and handle any planning permission applications. 

But it might simply come down to the scale of the project – the larger the project, the more benefit you might see from hiring an architect. 

We spoke to George Omalianakis from GOAStudio for his expert advice on working with an architect, and the key questions to ask when hiring one, and how much an architect costs.

If you're not yet a member, join Which? to get instant access to his tips on working with an architect.

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What about a structural engineer?

A structural engineer determines the strength and durability of a structure – ie a building. A key part of the construction process, they’re able to assess a building’s safety and will provide specifications and calculations for the design, as well as suggesting building materials, in building and renovation projects.

As a general rule, anything that changes the structure of the property will need building control approval. To obtain this you’ll need technical information from a structural engineer.

For a home renovation or inspection you’ll need a structural engineer with experience of residential work. This engineer will provide structural drawings and calculations for your building contractor or architect.

This information may also need to be provided to Building Control in order to comply with building regulations or, depending on the scale of the renovation, to planning officials in order to obtain planning permission.

Your engineer is likely to be a member of either the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE).

As with anyone involved in your project, it’s important to make sure your structural engineer has professional indemnity insurance.

Do I need a party wall surveyor?

You don't necessarily need a party wall surveyor for a home renovation. It depends on the complexity of your project in relation to the wall and/or the co-operation of your neighbour. 

Party wall agreements are most commonly needed for loft conversions and extensions which require the insertion of steel supports, a damp proof course or new foundations.

Read our FREE guide to building regulations and planning permission for further information on party wall agreements 

What about architectural technologists?

Similar to an architect, a chartered architectural technologist is qualified to offer full architectural design services and lead and manage projects from inception to completion. 

They specialise in design underpinned by their knowledge of building science, engineering and technology, playing a pivotal role in the construction process.

A chartered architectural technologist will come from a BSc(Hons) background and an architect will have a BA(Hons) background. 

The Chartered Institute of Architectural Technologists (CIAT) has further information on architectural technologists.

Visit our guide to extension costs for the latest RICS data on single and double-storey extensions

Looking for a reliable tradesperson near you? 

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