Understanding the Cost of House Extensions in the UK

1. What are the primary factors that influence the cost of a house extension in the UK?

The primary factors are: the type of contractor you decide to use, location, overall level of specification and the amount of remedial work to the existing building.

The amount of demolition and rebuild of certain elements to an existing house can add significantly to the amount of time needed on site and therefore cost increases in labour. For example re-doing a whole roof can be labour intensive and disruptive to other areas of the build. A loft conversion for example can become costly due to the potential need to install a temporary metal canopy for protection which can be upwards of £10k in London.

2. How do regional differences (e.g., London vs. rural areas) affect extension costs?

We think simplifying cost based on location can be misleading – sometimes we find in very rural areas there is a lack of availability of skills and labour which leads to a lack of competition. In London there are of course general uplifts in cost but these are not as significantly different to areas in the south and east of England generally.

3. What’s the typical price range clients should expect for a basic extension project?

A very basic ballpark is £2,500-3,000 per m2 for an extension but this is a very simplified guide. What is more important is to assess the building in its entirety and to understand how the proposed works affect other parts of the building. We generally advise and assist with initial pricing of the project around planning stage to give some clarity to our clients.

Inflation in recent years has meant that single storey extension projects have become more expensive though we have also seen people putting more energy and resource into their homes since the pandemic. The theme of wellness and ‘liveable’ buildings is increasingly in people’s minds and this leads people to prioritising the space in which they spend most time.

4. How do material choices, such as timber vs. brick, impact the overall budget?

This really depends on the type of existing building we are working with. Timber can be inexpensive but also can age poorly if not specified correctly. Brick will last forever but we always want to be involved in the specification of this as the quality and finish of a brick can vary enormously. Most building projects you see on a domestic scale use a generic brick which can be easily sourced from a local merchants but there is so much variety and nuance to how brick can be used.

As designers, we need to consider how the material choice will impact the demand on labour on site. Timber is a more versatile and flexible material to use – for example on our Aylsham project we are using timber as cladding for both the walls and roof of the extension. This is far from standard practice but in this case shows off the pre-treated timber as the focus for the project.

5. Can you explain how planning permissions or building regulations influence costs?

In the grand scheme of things for a building project, the cost of these in terms of the fees paid to the council or Building Control authority are small. While planning fees have increased of late to £613 for a Householder application, it is the same price as a ‘Certificate of Lawful Development’ so if you want documentation which will prove your development is legal in a planning sense, these costs need to be factored in. We always advise our clients of all the additional fees in our fee proposals.

We often find at an early stage in the project clients are wary of not wanting to get bogged down in the planning process which is understandable. At the same time, unless the development being proposed is clearly pushing the limits of what is reasonable, planning permission for an extension something we have had almost 100% success with.

Likewise Building Control is something which you are obliged to have for any building project. We can advise and provide contacts for companies which provide this service. For total clarity, we as designers are the ones who create the drawings and information which will be assessed by the Building Control officers as well them undertaking site visits as the build progresses.

6. What unexpected expenses commonly arise, and how can homeowners plan for them?

We would advise factoring in a contingency of around 10% for a building project. Likely expenses may include potential delays due to unforeseen issues when uncovering an existing building. For example on one project we discovered when repairing the roof on a terrace that part of the neighbours roof actually extended onto the client’s property. In order to avoid creating damage to the neighbours’ roof a Party Wall Agreement was required to set out exactly what work was required. This created a delay and impacted other works to the project but this could not have been found without the remedial works taking place.

7. How do you help clients decide between a fixed-fee contract and a cost-plus arrangement?

Our fees are generally fixed fee unless the job is very small where it might be appropriate to price based on a time charge basis. We don’t use the percentage of overall construction cost method as we believe this gives the impression that there is an incentive to drive up overall cost for the project. We price our fees based on the time and resource we need to undertake the project. As a RIBA Chartered practice our costs as a business are generally higher than a non-chartered practice of the same size as we have membership commitments and obligatory training and development courses which we have to take as a practice. We also see ourselves as providing a high level of service which creates a dialogue with our clients and not just drawing up a ‘bare-minimum’ design.

8. How might sustainable features, like green roofs or solar panels, affect the total cost?

These items can add cost but in general green roofs or solar panels can be added or removed towards the end of the project depending on budget considerations. Sometimes green roofs may be required as part of a pre-commencement planning condition but these do have real benefits with regard to slowing down water flows, providing benefits in terms of aesthetics and biodiversity and also some level of insulation to the building.

9. Can you break down the cost differences between a kitchen extension vs. a living room extension?

To us as architects the only difference would be how the space is fitted out. The costs of a kitchen can range dramatically depending on size, technology, materials, modular vs bespoke and so on. We work with kitchen companies who we feel respect the essence of the original design and people we can work with rather than just hand over responsibility. We have had great experiences working with brands such as Inglis Hall and Naked Kitchens.