Prefab lean-to extensions
A lean-to has a single sloping roof that rests against the back or side wall of the house. Three walls and one roof plane make it the easiest shape to prefabricate. Built and finished, allow £2,200 to £2,800 per m² including VAT. A 12 m² lean-to is £26,500 to £33,500.
Most are small: a utility room, a larger kitchen, a downstairs shower room or a boot room beside the back door.
12 m² lean-to, built and finished
£26,500 to £33,500
About 4 m by 3 m, including VAT.
A lean-to is a good answer if
- You want 8 to 15 m² more floor and no more.
- There is a blank stretch of wall below the upstairs windows.
- A side return is sitting unused beside the kitchen.
- You would like a proper insulated room where a tired conservatory stands.
- The budget rules out a full-width extension.
Prefab lean-to prices
| What | Typical price including VAT |
|---|---|
| Built and finished, per m²Insulated walls and roof, plastered, wired | £2,200 to £2,800 per m² |
| 8 m² side returnAbout 5.5 m by 1.5 m | £17,500 to £22,500 |
| 12 m² rear lean-toAbout 4 m by 3 m | £26,500 to £33,500 |
| Lean-to kit, panels onlyPer m², nothing fitted | £1,200 to £1,700 per m² |
| Strip footings on firm groundThree sides only; the house is the fourth | £1,000 to £3,000 |
Typical national ranges for 2026, including VAT; London and the South East usually cost 20 to 40% more. Small rooms sit towards the top of a per m² range, and sometimes above it, because fixed costs are spread over less floor. A guide, not a quote. Try your own size in the calculator
The roof pitch decides the design
The top of a lean-to roof has to stay below the upstairs window sills, and the bottom has to clear the doors. On most houses that leaves about a metre of rise. Over a 3 m depth, a metre of rise gives a pitch of roughly 18 degrees. Over 4 m it drops to about 14.
Ordinary roof tiles need more slope than that, often 22.5 degrees or more, and plain tiles far more. At these pitches you are choosing from low-pitch tiles, standing-seam metal or a single-ply membrane. Insulated roof panels suit shallow pitches well, which is one reason prefab and lean-to go together.
If the sums do not work, lower the eaves, shorten the depth or accept a flat roof with a rooflight.
Planning limits
In England a single-storey lean-to on the back of a house is usually permitted development if it stops within 3 m of the back wall of an attached house, or within 4 m where the house is detached. It must be no taller than 4 m, with eaves no higher than 3 m when it stands within 2 m of a boundary. A side lean-to can be no wider than half the width of the original house (Planning Portal).
None of this applies to a flat or a maisonette, where every extension needs permission. In Scotland the measurements are similar but the tests differ, and you need a building warrant before starting.
Extension or conservatory?
The difference matters for building regulations. A conservatory with a floor below 30 m² escapes most of the regulations, provided external-grade doors shut it off from the house and it is heated separately. Remove those doors, or build with solid insulated walls and roof as part of the house, and it is an extension that must pass in full.
For a lean-to extension in England that means walls at 0.18 W/m²K, a roof at 0.15 and a limit on glazing: windows, doors and rooflights together should not exceed a quarter of the new floor area plus the area of any openings the extension covers, unless your designer shows the heat loss is made up elsewhere (Approved Document L).
Side returns and drains
The strip of yard beside a Victorian kitchen is a classic lean-to site, and it nearly always has a drain run or an inspection chamber in it. Building over a public sewer needs the water company's consent. Find the chambers before you fix the layout; moving one costs far more than moving a wall on paper.
A wall on the boundary within a metre of next door also has to resist fire, which limits timber cladding and windows on that side.
Know the wall you want to build against? Send its length and we will look for a firm to price the lean-to.
Get a quoteBuilding a prefab lean-to
Measure the wall
Sill heights, door heights and drain positions set the roof pitch and the depth.
Footings on three sides
Dug and poured in a few days while the panels are made.
Panels and roof
Walls stand in a day or two. Roof panels fix to a timber plate bolted to the house.
Flashing and fit-out
Lead or metal flashing seals the join. Then the opening, services and finishes.
Lean-to questions
How much does a prefab lean to extension cost?
Around £2,200 to £2,800 per m² built and finished, including VAT. A 12 m² room comes to £26,500 to £33,500. Very small rooms cost more per m².
What is the minimum pitch for a lean-to roof?
It depends on the covering, and the maker of the tile or sheet sets the figure. Many standard concrete tiles want 22.5 degrees or more, and plain tiles much steeper. Low-pitch tiles, metal sheet and insulated roof panels go lower, and a membrane roof can be nearly flat.
Does a lean-to extension need planning permission?
Often not, if it is single storey, within the 3 m or 4 m depth limit, under 4 m high and built in materials similar to the house. Conservation areas, flats and listed buildings are different.
Can a lean-to replace my conservatory?
Yes, and the old base can sometimes be reused if an engineer confirms the footings are deep enough. Conservatory bases are often too shallow for a solid, insulated room.
Can I build a lean-to up to the boundary?
You can, with care. Gutters and foundations must not cross the line without the neighbour's agreement, and a party wall notice may be needed (GOV.UK).
Keep reading
- Prefab extension kitsSmall kits and what they include
- Cost calculatorPrice a small room
- Flat pack extensionsPanels carried through a side gate
Get a quote for a lean-to extension
Postcode, four quick questions, then your contact details. It takes about a minute.
We are an introduction service, not a prefab extension builder. We search for a company that takes work near you and give you its name first. Your details stay with us unless you agree to the introduction, and asking is free.
That is everything. Your request is with us.
- We look for a firm that builds the kind of extension you chose and covers your postcode.
- We email or phone you with that firm's name. You decide whether they get your details.
- If you say yes, they arrange to see the house and price the work. We never send marketing.