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.

Find out how much an extension costs – we've got the latest figures from the Building Cost Information Service to help you budget

How to plan a home extension: 5 essential first steps

1. List your must-haves and nice-to-haves: Before speaking to a professional, write down exactly what you need. Is it a bigger kitchen, or a dedicated home office? Knowing your non-negotiables prevents ‘scope creep’ later.
2. Set a realistic budget: Don’t just think about the build cost. You need to factor in VAT (20%), professional fees (architects, structural engineers), and a 10–15% contingency fund for unexpected delays or discoveries.
3. Check your permitted development rights: You might not need full planning permission. Research if your project falls under permitted development (PD), which can save months of waiting for council approval.
4. Survey your site: Know where your drains are and where your boundary lines sit. Moving a sewer pipe can add thousands to a quote. Hiring a surveyor can help avoid any issues like this. 
5. Research the right professionals: Decide if you want an all-in-one design-and-build firm, or if you prefer to hire an independent architect and then approach different building companies for quotes to complete the work.

How to find a reliable builder

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 builders.

Check that the builder is a member of a reputable trade body, such as the FMB or an endorsement scheme, such as Which? Trusted Traders. 

Which? Trusted Traders raises standards for consumers by setting a benchmark that all traders need to meet, and every customer review is verified as genuine by moderators. Use the tool to find a reputable builder near you: 

Top tips from a builder

Joseph Cole, director of All Build Developments, a Which? Trusted Trader, 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.

Pricing and payment

The first mistake most people make is choosing a contractor based on price. Choose one based on trust and professionalism, not how much it will cost you.

Don’t get hung up on fixed price. Obvious stuff like the fabric of an extension is easy for a builder to work out and give a fixed cost for. Fiddly work like an expensive kitchen or some fancy flooring can be harder to cost and considered a high risk item. Why not get a preliminary sum for such items from your builder for your budget then have a day work or cost plus agreement to do that part of the work. You will get the right job at a fair price and the builder will have covered his costs fairly.

Make sure you agree a payment plan that ties in with your project plan.

Don’t offer your builder cash; it encourages low standards in the industry and under the radar cowboy builders.

Getting quotes

More than three quotes and you are wasting peoples time. Why not get three rough estimates from three builders, pick your favourite two, and then get them to quote properly? 

You will get far more accurate quotes if the builder knows he is in the running against the other one.

Get a written specification. The quote should come on headed paper and itemise everything that is included. It is also important to list what isn’t included. For example, if there is something the client proposes to do themselves.

Give your builder time

Remember most of us are busy with our hands in the day so quoting can take a while to catch up with and is often done during the evenings or weekends. Give them a minimum of a fortnight to quote and don’t expect an immediate start; a good builder should have three or four weeks work ahead of them in the diary.

Check references and qualifications

Get references and follow up on them. We actually insist our new clients come to a current job and meet an existing client. That should tell you a lot about who you are about to use.

Does your builder have a qualification? And be sure to check your builder's insurance details.

Is the builder a member of a reputable trade body? The best place to start is the FMB or Which? Trusted Traders as these builders have all been vetted and inspected.

Prepare a spec

Builders aren’t mind readers. A room by room spec and pictures will help them understand what you want to achieve.

Get a contract

Go through with your architect to administer the contract for you or agree a written contract between you and your builder. FMB registered builders have access to standard contracts that are watertight and cover both parties in the event of a breakdown of relationship. If you do everything preceding this point, then that won’t happen!

What to look for when hiring an architect

There are two key institutions that support the architecture profession in the UK: the Architects Registration Board (ARB), which is the regulatory body for architects in the UK, and the Royal Institute of British Architects (RIBA), a chartered body dedicated to promoting excellence in architecture. 

Both maintain registers of qualified professionals that you can use to find an architect to work with. See architects-register.org.uk and find-an-architect.architecture.com.

Top tips from an architect

We spoke to George Omalianakis from GOAStudio for his expert advice on working with an architect, and the key questions to ask before you start working with them. Find out about previous projects and their approach to managing the process, from understanding your priorities to dealing with the builders and other construction professionals involved with your project.

Key questions you should ask in advance of an appointment are:

• How they calculate their fees
• What are the terms of their appointment
• How many design options and site visits they include in the fee proposal
• What other services they offer
• Who does the design work in their practice
• What will be the level of their personal involvement in your project

Will the architect sort out planning permission?

Yes, an architect will handle your planning application, but that doesn't necessarily guarantee approval for your plans. 

Architects can foresee what the planners might approve for your project but they cannot guarantee an approval; it is for the local authority to assess an application, consider it in the context of the local planning policy, evaluate the immediate physical context and take a view on the design merit of a proposal and the validity of any objections received. 

Your architect can advise on what is and isn't likely to get approval, and with that knowledge decide whether it's worth applying for something more adventurous, or go for something safer that's likely to get timely approval. Your specific circumstances will determine the best way forward and your architect will then proceed as necessary.

How much does an architect cost?

If you wish to hire an architect, their fees are usually around 15%, so you’ll need to factor this into your budget.

When assessing potential building costs, consider a reasonable budget for the works and discuss it with your architect to see if it is realistic.

When starting the design development stage, make some initial decisions about the design proposals and the extent of the alterations you wish to carry out. At this point, obtain rough cost estimates from a builder, or use a cost consultant for something more detailed. 

If the estimates are around a figure you're comfortable with, then start preparing the required technical and structural information for the planning application. Then ask the cost consultant or the builder to provide you with more accurate and reliable estimates.

If the estimates are still within your budget, then proceed with obtaining planning permission and prepare the detailed tendering information so that builders can provide detailed quotes. If at any point the estimates are outside your comfort zone, change the design as necessary.

Remember: before obtaining costs, you need to prepare a list of what you need priced up.

How long will the process take?

For a typical residential extension that requires planning and building regulations approvals and a party wall agreement, allow a minimum of five to six months between appointing your architect and the building works starting. 

Invest time on the design development stage and on the selection of your builders. You will have to wait for fixed periods of time to get some of the approvals (typically eight weeks), and good builders are often booked up months ahead.

What about my neighbours?

Your neighbours can object to a planning application, and they often do so. Always have a chat with them before you make the planning submission. 

Unless legal or ownership matters are at play, your neighbours cannot really stop your project if the proposals are otherwise acceptable to the planners and within planning policy. 

During party wall negotiations, your neighbours can potentially delay the process until an agreement is reached under the Party Wall Act, but they can not effectively stop your project.

Any other things to consider?

Create inspirational moodboards of your planned extension to help get your ideas across to an architect. Do this by collecting images of things you like by tearing out pages from magazines or creating a Pinterest board online. 

With your budget in mind, consider adding sustainability measures such as solar panels, a heat pump or a green roof. Check ownership issues, such as who owns the air above the roof you want to extend or who owns the roof / loft space if you live in a top-floor flat.

You might also want to consider the following questions: 

• How long do we plan to live in the property?
• How is our lifestyle likely to change over this time? 
• What are the chances for a growing family or working from home?

Find out if a heat pump would work for your home

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.

Our guide to building regulations and planning permission offers 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.