How Much Does a Rear Extension Cost in the UK? A Room-by-Room Breakdown

Ask five builders to price the same rear extension and you will get five different numbers. That is not usually because four of them are chancing it. A rear extension is a bundle of separate jobs — excavation, foundations, structure, roof, glazing, heating, finishes — and a small decision in any one of them shifts the total. Understanding where the money actually goes is the difference between a budget you control and one that controls you.

Below is a realistic breakdown of what rear extensions cost in the UK, how that changes room by room, and the fees that rarely appear on the first quote.

What actually drives the price

Most builders price by the square metre of floor area, then adjust. As a rough guide for a straightforward single-storey extension in average ground conditions:

• Standard specification: around £1,500–£2,500 per m²
• Double-storey: around £1,800–£2,800 per m², because you are paying for two sets of walls, floors and finishes but only one excavation
• High specification: £3,000+ per m² once you add structural glazing, a large sliding door, underfloor heating and a designer kitchen

London and the South East sit at the top of those ranges; parts of the North, Wales and Scotland often sit at the bottom. Ground conditions matter more than most people expect — clay, tree roots close to the foundations, a high water table or a sewer running across the footprint can add thousands before anyone has laid a brick.

Rule of thumb: the smaller the extension, the higher the cost per square metre. Fixed costs — excavation, scaffolding, the roof, the skip — do not shrink with the floor area.

Typical rear extension costs by size

These figures assume a single-storey extension at a mid-range specification, excluding kitchen units and appliances. Treat them as a starting point for conversations, not as quotes.

• 12 m² (roughly 3m × 4m): £25,000–£40,000
• 20 m² (4m × 5m): £35,000–£55,000
• 30 m² (5m × 6m): £50,000–£85,000
• Two storeys, 20 m² footprint (40 m² of new space): £75,000–£120,000

Add a kitchen and those numbers climb quickly. A fitted kitchen alone commonly runs from around £8,000 for a modest range to £30,000 or more for a bespoke one, and it is usually supplied separately from the building work.

A room-by-room breakdown

Kitchen-diner extension

The most requested — and the most expensive — option. Structure is only part of it: you are also paying for extractor ducting, plumbing for a sink and dishwasher, more sockets than any other room in the house, splashbacks, flooring that can take a spill, and often a structural opening back into the original house. Budget £45,000–£85,000 for a 20–30 m² space including a mid-range kitchen, and more if you want a lantern roof or sliding doors.

Living room or garden room

Cheaper because the services are simpler. A 15–20 m² garden room with a large opening to the garden, a couple of radiators and engineered flooring typically lands between £30,000 and £55,000. Spend the savings on glazing — it is the single element that decides whether the room feels generous or gloomy.

Utility room, boot room or downstairs WC

Small extensions of 6–10 m² are poor value per square metre because the fixed costs stay the same, but they can transform how the rest of the house works. Expect £18,000–£35,000. A WC adds drainage and ventilation costs; a utility needs plumbing, drainage and, ideally, its own door to the garden.

Bedroom and bathroom above

A double-storey extension spreads the foundation and roof costs over two floors, which is why 40 m² of space often costs less than twice what 20 m² would. An upstairs bedroom with an en suite typically adds £35,000–£60,000 on top of the ground-floor element, largely because of bathroom drainage, tiling and waterproofing.

Specification choices that move the total most

If you need to trim a budget, these are the levers worth pulling:

• Glazing: aluminium bi-fold or sliding doors cost noticeably more than uPVC, and they need a stronger — sometimes steel — lintel above.
• Roof: a flat roof with a roof lantern is usually cheaper than a pitched, tiled roof, but it needs careful insulation and drainage detailing.
• Heating: extending your existing system is cheaper than underfloor heating, which also raises floor levels and can complicate door thresholds.
• Flooring: porcelain tiles over a large area add up fast. Engineered wood or luxury vinyl is often a better cost-to-look ratio.
• Moving services: relocating a boiler, consumer unit or soil pipe is fiddly, time-consuming work that always costs more than expected.

The hidden fees nobody mentions at the first meeting

Building work is rarely the only line on the invoice. Set aside a contingency of 10–15% of the contract value, and budget separately for:

• Planning application: many rear extensions fall under permitted development, but a householder application still carries a fee set by your local authority — check the current figure on their website.
• Building control: inspections and the completion certificate are charged separately, typically several hundred pounds.
• Architectural drawings and structural calculations: usually a few hundred to a couple of thousand pounds depending on complexity.
• Party wall agreements: if you are excavating near a neighbour's boundary, you may need notices and a surveyor — budget roughly £700–£1,500 per affected neighbour.
• Drainage: a build-over agreement or a diverted sewer can be a significant, unpleasant surprise.
• Permits: skip licences, scaffolding licences on a public highway, and any parking suspensions.
• Temporary arrangements: storage, a makeshift kitchen, eating out, or moving out for a week or two.
• VAT: most extension work is standard-rated. Always ask whether a quote includes VAT — a "cheap" quote that excludes it is not cheap.

Planning rules, party wall procedure and building regulations differ across England, Scotland, Wales and Northern Ireland. For anything involving a boundary, a shared wall or a listed building, take professional advice before you commit.

How to keep the budget on track

Get the drawings and specification as complete as you can before you ask for prices — changes made once work has started are the most expensive changes of all. Fix a payment schedule tied to completed stages rather than to dates. Keep a written record of variations, signed by both sides, and hold back a small retention until snagging is finished. And ask each builder what is excluded, not just what is included; that is where the real differences between quotes live.

Getting quotes you can compare

Approach at least three builders, give them identical drawings and the same specification, and ask for a fully itemised quote rather than a single lump sum. Check references from a project finished at least a year ago, confirm their insurance and, if you are in England or Wales, whether they are registered with a competent person scheme for any electrical or gas work. A cheaper quote from someone who cannot show you a similar completed extension is not a saving — it is a risk you are taking on their behalf.

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