DATA-BACKED GUIDE · UPDATED JULY 2026
Single storey extension cost UK: the per square metre reality
A single storey rear extension is the classic way to open up a kitchen or add a bright family room. Builders often quote a rough per square metre rate, but that number hides as much as it tells you. The real prices below come from actual prices, so you can sense-check any quote against what homeowners have genuinely paid.
The quick version
- Per square metre rates are a starting point only. Kitchens, bifold doors and groundworks can move the true figure well above the headline rate.
- Single-storey rear extensions usually fall under permitted development within the size limits, so full planning permission is often not needed.
- A bigger footprint does not scale the price evenly, because the fixed costs of foundations and a roof are spread across more floor area.
- Fit-out choices like flooring, glazing and units often cost as much as the shell, and they are where budgets quietly overrun.
What people actually paid
Real prices, in people's own words
- £45,000“Single storey 6.5m wide x 4m long (2022) peak inflation material costs. Included 4 keylite windows and 8ft bi fold. Gable roof. £45k”
- £51,816“Total cost £51,816 (give or take a couple of quid)”
- £80,000“We just finished a single storey. It was £2.5k per square meter plus VAT. Cost 80k in total”
- £95,000“Approx 95k, 5x3m. Included a new kitchen, downstairs WC, exterior power, knocking together upstairs bathroom & WC to make 1 room and unexpected new electrical works to most of the downstairs.”
- £95,000“Total cost £95k, of which build cost was £65k. Kitchen & utility £15k, flooring £3k, architect and planning £2k”
- £100,000“Our recent 2 storey side extension was upwards of £100k building costs.”
Genuine amounts posted publicly. We publish the price and the quote, never the person.
Why the price varies so much
The per square metre figure only ever covers the basic shell. Once you add a kitchen, a roof lantern, bifold doors, underfloor heating or a knocked-through wall with steel, the real cost climbs. Ground conditions matter more than people expect, since poor soil or nearby trees can mean deeper foundations. Location plays a part too, with labour in the south east typically dearer than elsewhere. The finish level is the big swing, because a plain plastered box and a fully fitted open-plan kitchen sit at opposite ends of the range even at the same square metre size.
How to pay less
- Ask for quotes broken down into shell and fit-out so you can see where the money actually goes and trim the extras.
- Keep the design a simple rectangle. Every extra corner, roof valley or step in the wall adds labour and material.
- Source your own kitchen, tiles and flooring where the builder allows it, rather than paying a markup on their supply.
- Get the drainage and foundation approach checked early so you are not hit with a costly surprise once the ground is opened up.
Common questions
Is a per square metre price reliable?
Treat it as a rough guide, not a fixed quote. It usually covers the shell only and leaves out the kitchen, glazing, groundworks and finishes that make up a big chunk of the final bill. Use the real prices below to sanity-check whatever rate a builder gives you.
Do I need planning permission for a rear extension?
Single-storey rear extensions often fall under permitted development as long as they stay within the depth and height limits. You will still need building regulations sign-off for the structure, insulation and drainage, which is a separate approval from planning.
Does the quote include VAT?
Most building work carries 20% VAT, so always check whether it is included before comparing prices. A quote without VAT will look cheaper on paper but cost the same once it is added.