How much will an extension cost?

The actual cost of an extension will depend on several factors, including the size, complexity, structural requirements, finish, quality, site access and significantly, where your home is located.

Based on information taken directly from our live projects and competitive tender returns we recommend that for new build extension works, including one and two-storey side returns, and rear and front extensions, you allow between £3,250 and £3,750 per square metre. This would include roof spaces (loft or attic conversions), however again the costs will depend on the size of your home and the extent of space you wish to achieve within your attic or loft space. If your attic is convertible ie. A dormer can be added, and the existing roof structure is largely retained and modified, this will be cheaper than a wholesale roof replacement. However, the latter will potentially afford you a much greater level of increased space.

Basements are the most expensive way to expand your home, we would suggest allowing between £4,000 and £5,500* per square metre.

*Subject to access and size.


It is also important to realise that any extension project that is to be fully integrated into your home, should consider the costs for the refurbishment of existing spaces. Again, specific costs are subject to the complexity of the refurbishment works, with a clear difference between simply painting a room or knocking a room through to another, introducing new structural steelwork, replastering, rewiring and then redecorating. For full refurbishment with structural change and new plumbing and electrical services, we would suggest allowing between £3,000 and £3,500 per square metre. For comparatively light touch refurbishment works, we would suggest allowing between £1,500 and £2,500 per square metre.

It is important to note that these square metre rates are simply a guide for build costs only, excluding VAT & professional fees. They are used at the early feasibility and concept stages of your architectural journey, to provide an indication of costs and inform budget management. As the project develops a cost professional (Quantity Surveyor) can use more precise methods of measurement to estimate the construction costs, factoring in any unique considerations of your project.

At RHJB we use our experience to guide you along your journey to achieving the luxury home you seek. With our utmost endeavours to achieve the best solution with cost transparency at all stages.

If you are considering a new build project, please refer to our blog “How much will a new luxury house cost?”

In an ever-changing landscape of costs, which has seen increases in the last year for various reasons, RHJB are keen to keep you updated. It is our intention to update these costings with both live data and information from cost professionals as and when we see significant changes in the market.

Last updated May 2023