See our full guide to building an extension

We don’t want to blind you with jargon and technical specifics with this guide, we just want to cut to the chase and give you some quick answers to three basic questions as a useful starting point. How much, how long and what’s included!


How much will it cost?

Guide costings vary across the country and you probably won’t be surprised to learn that London and the South East is the most expensive region. For a standard specification house extension, the typical cost in and around London is £1,700/m2 (significantly more than other areas in the country) – but, of course, that means nothing unless you understand what is generally regarded as a ‘standard spec’.


What do I get for my money?

A typical finish would include all everything necessary to build the extension (ground works, walls, roof etc) with a single window or door. Internally a standard specification would also include plastered walls, a double electric socket, light and switch and one radiator. In addition, to the basics of power, lighting and heating, you can expect skirting boards and architraves to be fitted and the floor to be prepared for the finish you require.

Additional items, not included in a standard spec, would include bi folding doors, roof lanterns and any extra lighting or heating requirements, such as underfloor heating or extra radiators, for example. Flooring finishes: tiling, carpets, etc. would be fitted for an additional cost. The other item is any strutural works required, such as opening up the back of a house where the extension is planned for.


How long will it take?

In general, the time to build a typical extension is 3 to 4 months. However, the actual time the total projects takes from ‘I think I want an extension” to it being built can take up to 9 months. There are three stages of the process: (1). Design, (2) Planning and (3) Build.

(1) Design: 1-2 months. That includes the initial consultations and drawing up of the outline design. A survey is then carried out, formal plans produced, and submitted to planning. At stage you can start to get some indicative ideas on costings.

(2) Approval: 3 months – plans are submitted to the council for approval. In some circumstances the strutural design is completed which is usualy the last piece of the equation to then finalise a quotation.

(3) Build: 3 – 4 months.

See our full guide to building an extension