The short answer
The cheapest extension is a plain rectangle with a simple roof, kept inside permitted development limits and away from the drains. Most of the cost sits in foundations, glazing and the opening into the house, so those are the places to save. Cutting floor area saves less than people expect.
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Where the money goes
Before trimming anything, know what you are trimming. A single-storey brick and block extension typically costs £1,800 to £3,000 per m², and a panel-built one £2,200 to £2,800. These are national ranges for 2026, including VAT; London and the South East usually cost 20 to 40% more. Neither figure is spread evenly across the floor.
A handful of items cost much the same whether the room is 12 m² or 25 m²: the design, the engineer, the opening into the house, the doors to the garden, the electrician’s first visit. That is why a small extension always looks expensive per square metre, and why shaving two square metres off the plan barely moves the total.
The ideas below go after the fixed costs and the expensive parts.
Shape and size
1. Draw a rectangle. Every corner adds foundations, roof detailing and labour. An L-shape or a wraparound costs noticeably more than a box of the same area.
2. Choose a simple roof. A flat roof with one rooflight is the cheapest way to cover a room. A lean-to with a single slope is close behind. Hips, valleys and lanterns are where roofers earn their money.
3. Go up, not out, if you need two rooms. Two storeys share one set of foundations and one roof. They typically cost £1,700 to £2,900 per m² of total floor, which is often less per room than two single-storey additions.
4. Use a side return. The strip beside a terraced kitchen is already walled on two sides. Filling it adds a surprising amount of usable floor for the size of the build.
Planning and paperwork
5. Stay inside permitted development. In England that usually means a depth of 3 m behind an attached house or 4 m behind a detached one, and a height of 4 m at most. You skip the £548 application fee, the drawings it needs and the eight-week wait. A lawful development certificate, at £274, gives you proof for when you sell.
6. Finish the design before the builder starts. Changes on site are the most reliable way to overspend. Fix the layout, the sockets and the door positions on paper, where moving them is free.
Structure
7. Keep clear of drains. Building over or near a public sewer needs the water company’s consent and sometimes a diversion. Moving the extension half a metre on the drawing can save thousands in the ground.
8. Reuse an existing opening. Patio doors or a wide window already have a lintel above them. Using that opening as the way into the new room avoids a new steel beam, the engineer’s design for it and a week of propping.
9. Ask about foundations early. On firm ground, strip footings typically cost £1,000 to £3,000. Clay soil, nearby trees or made-up ground can call for a raft or for piles, which your engineer designs and prices after a ground investigation, and which can add several thousand pounds. A trial hole before you finalise the design costs little and removes the biggest unknown.
Finishes and method
10. Be careful with glass. A full wall of sliding doors is often the costliest single item. A 3 m set of doors and a fixed window gives nearly as much light for far less. Stock sizes cost less than made-to-measure.
11. Buy a shell and finish it yourself. Some firms will build a weathertight, insulated shell and leave. If you can paint, lay flooring and fit a kitchen, that is real money saved. A prefab extension bought as a kit takes this further: panels for a 25 m² room cost £30,000 to £42,500 including VAT, and you organise the rest. It only saves money if you can manage the trades, so be honest about your time.
Savings that are not worth it
Some cheap extension ideas cost more in the end.
Skipping the structural engineer is one. Building control will ask for calculations anyway, and by then the steel may be in.
Thin insulation is another. Walls in a new extension must lose no more heat than 0.18 W/m²K in England. A room that misses it will fail inspection, and a room that only just passes will cost more to heat every winter.
Paying cash to dodge VAT leaves you with no contract and no comeback. A VAT-registered builder must charge 20% on an extension to an ordinary home. A small builder below the VAT registration threshold does not charge it, which is legal, but that is the only honest way the bill comes without it.
And the lowest quote is not always the cheapest job. Compare what each builder has included. A price that leaves out the plastering, the electrics or making good the garden is not lower, only shorter.
A sensible order of attack
Start with the shape and the planning route, since they are free to change. Then get the ground investigated. Then set a glazing budget and stick to it. Leave finishes to last: they are the easiest things to upgrade later, when the pain of the build has faded.
To see what your size might cost by each build method, try the cost calculator, or ask for a price using the form below.