What does a side return extension cost in London?
In London: £30,000 to £45,000 for a typical 12 m2 project at mid-range, against £24,000 to £36,000 nationally. Budget starts around £25,500, premium runs to £54,000. Set your own size, finish and region below, no email required.
What the sources say
Checkatrade, Side Return Extension Cost: £1,500 to £4,500
per m2 table £1,500-4,500 (average £3,000); the text says £1,500-3,000 per m2 outside London and up to £4,500 in London; a small single-storey side return £30,000-45,000. The guide does not state its VAT treatment. Guide dated 2026-09, opened and checked 2026-09-29.
The national range at the top of this page is chosen inside the spread these guides publish, with 20% added where a guide states its prices exclude VAT. Where guides disagree, both figures stay visible above so you can judge for yourself.
Side return extension, mid-range, London
£30,000 to £45,000
Ask each trade whether VAT comes on top. An indication, not a quote: prices vary per trade and property.
Take it further
Nothing here is saved unless you ask. With a free account you can keep this estimate and come back to it later.
Where a guide says its prices exclude VAT, I added 20%. Most guides don't say, so ask each trade whether VAT comes on top. A trade under the £90,000 VAT threshold is often not registered and then charges none.
On top of whatever you agree with a builder, set aside a contingency of 20% for the unforeseen: structural work on the existing building carries the most unknowns; guidance puts these projects at the top of the contingency range or above. How much contingency you need explains the guidance.
Extensions and loft conversions often fall under permitted development. Check before you budget.
What drives the price
- Working in a narrow strip: access and logistics make the per-metre price the highest of all extensions.
- The roof: glazed or part-glazed side return roofs are where the drama and the money sit.
- Drains: the side return often hosts the original drains, which then need moving.
- The party wall: terraced and semi-detached side returns usually trigger party wall agreements.
All figures come from the same validated 2026 price data as the calculator above, built on the sources listed higher up on this page, each opened and checked on 26 August 2026 (see how we price), so this page and your estimate can never disagree.
Outside London? See the national guide, or compare every region in the regional overview. How to pay for it (remortgage, loan or savings) is its own guide: paying for a renovation.
Planning an extension or loft? Check permitted development first.
Frequently asked questions
Why is a side return so expensive per square metre?
You are buying a small number of square metres with all the fixed costs of a full extension: foundations, steels, roof, glazing and drain moves in a tight strip with awkward access. The value is in what it does to the whole ground floor, not in the metres themselves.
What should I do first?
Three steps, in this order. First, turn the guide range above into your own number: the calculator on this page adjusts for size, finish level and region in under a minute, free and without email.
Second, if the layout matters, bathrooms and kitchens especially, draw the room to scale in the free planner; moving plumbing is usually the biggest hidden cost, and you want to know that before anyone quotes. Third, get at least three itemised quotes and compare them line by line against your range before you commit; the quote check on this site gives you a verdict per quote.
Do these prices include VAT?
Not for certain. Where a guide says its prices exclude VAT, I added 20%. Most guides don't say, so ask each trade whether VAT comes on top. A trade under the £90,000 VAT threshold is often not registered and then charges none. New builds and some energy-saving works can qualify for 0% or reduced VAT; ask your installer whether your project qualifies.
Why is London so much more expensive?
Labour is the driver: London trades charge roughly 25% above the national average, and parking, access and waste logistics add friction. Nationally this project runs £24,000 to £36,000; the calculator switches region in one click.
How accurate is this range?
It is a guide range from national 2026 cost guides, adjusted for finish level and region. Real quotes vary by trade, property and specification; treat it as a starting point, budget 10 to 15 percent contingency, and always get at least three itemised quotes.