Can I build an extension for 20k?
Not a habitable extension in London. £20,000 in Fulham covers a garden room, a small porch, or a conservatory — structures that avoid full Building Regulations. A heated, insulated extension connected to the house starts at roughly £45,000 to £50,000 even at a small footprint.
The reason is that a habitable extension carries fixed costs that do not shrink with size. Foundations to the depth London clay and nearby trees require, a designed structural opening back into the house, a party wall award, building control fees, drainage connections and professional fees all have to happen whether the extension is 8m² or 25m². Those fixed costs alone approach £20,000 on a Fulham terrace.
What £20,000 does buy is a garden room — an insulated timber-framed structure, independent of the house, under 30m² and outside Building Regulations if it is genuinely detached and unheated by the main system. For a home office or a studio this is often a far better use of £20,000 than a token extension, and it can usually be built under permitted development.
A conservatory is the other option in that bracket, though the picture has changed: a conservatory only stays exempt from Building Regulations if it is separated from the house by external-quality doors and has an independent heating control. Knock through into it and it becomes an extension in the eyes of building control, with all the requirements that brings.
We would rather say this plainly than quote something that cannot be delivered. If a builder offers a full habitable extension in London for £20,000, look closely at the foundation depth, the insulation specification and whether building control has been engaged at all. Those are the three places corners get cut, and all three are expensive to put right later.
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