How much does a home extension really cost?

Ask a builder how much an extension costs and the honest answer is usually "it depends." Size, specification, location, ground conditions and finish quality all move the figure. That is not evasion — it is reality. A standard rear extension in Berkshire is a different proposition from a complex wraparound build in West London.

This guide gives you realistic ballpark figures for UK home extensions in 2024, explains what drives the price and shows where the hidden costs hide. If you want help locking in material costs, request a quote from Star Hawk Builders Merchant or find your nearest branch.

Build cost per square metre

The simplest way to estimate an extension is by cost per square metre. These figures include labour and materials for a typical specification, but exclude professional fees, VAT surprises and high-end finishes.

Type of extension	Cost per m²	Typical total
Single-storey rear extension	£1,800–£2,500	£36,000–£75,000
Two-storey rear extension	£1,500–£2,200	£60,000–£110,000
Side return extension	£2,000–£3,000	£40,000–£90,000
Wraparound extension	£1,800–£2,800	£80,000–£150,000+
Garage conversion	£1,000–£1,800	£15,000–£35,000
Loft conversion	£1,500–£2,500	£40,000–£75,000

These are broad ranges. A basic single-storey extension with standard windows, a flat roof and modest finishes will sit at the lower end. A large open-plan kitchen extension with bifold doors, a pitched roof, underfloor heating and a high-end kitchen will push well past the upper end.

What drives the cost up or down

Several factors have a bigger impact on price than most homeowners expect.

Size and shape

A simple rectangular box is cheaper per square metre than an L-shaped or angled design. Every corner, bay and change of roofline adds labour and material complexity.

Foundations and ground conditions

Normal strip foundations are relatively cheap. If the ground is poor, the water table is high or there are trees nearby, you may need deeper or reinforced foundations. This can add thousands before the walls go up.

Roof type

A flat roof with a basic membrane is cheaper than a pitched roof with tiles. Lanterns, rooflights and sliding glass roof panels add cost but transform the space.

Openings

Standard windows and a single door are inexpensive. Large aluminium bifolds, sliding doors or corner-opening glass walls can cost £5,000–£15,000 or more depending on size.

Kitchens and bathrooms

A builder's quote usually stops at plastering. Fitting out a kitchen extension with units, worktops, appliances, lighting and flooring can easily add £10,000–£30,000. A bathroom or utility room adds another £3,000–£8,000.

Location

Labour and material costs are higher in London and the South East than in the Midlands, North or Wales. Expect a 10–20% premium inside the M25.

Access

If scaffolding has to bridge a conservatory, materials must be carried through the house or there is no parking for deliveries, expect extra labour costs.

The professional and statutory fees

The builder's quote is only one line in your budget. You also need to allow for:

• Planning permission or Lawful Development Certificate. A householder planning application costs £462 in England. A Lawful Development Certificate costs around half that and is useful when you sell.
• Building regulations approval. Almost all extensions need this. Fees range from £500 to £1,200 through your local authority or an approved inspector. You can read more on GOV.UK.
• Architectural drawings. Expect £1,000–£3,000 for plans and £500–£1,500 for a measured survey.
• Structural engineer. Steel beams, load-bearing wall removals and complex roofs need calculations, usually £400–£800.
• Party wall surveyor. If your work affects a shared wall and your neighbour does not consent, budget £700–£1,500 per neighbour.

These fees routinely add £3,000–£8,000 to the project cost before ground is broken.

On-site extras that add up

Once work starts, several costs appear that are not always in the builder's quote:

• Skips. A typical extension needs 3–6 skips at £250–£400 each.
• Scaffolding. Simple single-storey access might be £500–£1,000. Two-storey or complex scaffolding can exceed £2,000.
• Temporary services. Rerouting electrics, water or gas during the build can cost £200–£1,000.
• Surveys. Tree surveys, asbestos surveys and drainage CCTV surveys may be needed, especially on older properties.

For a fuller list, see our guide to hidden costs that blow your extension budget.

Finishing costs after the builder leaves

The builder's quote often ends at plastering. After that, you are responsible for:

• Painting and decorating.
• Floor finishes — tiles, wood, carpet or vinyl.
• Kitchen units, worktops and appliances.
• Bathroom fittings.
• Light fittings, switches and sockets.
• Curtains, blinds and furniture.

These finishing costs can easily add £10,000–£30,000 to a kitchen extension, so build them into your budget from the start.

A realistic example budget

Here is what a typical £60,000 single-storey kitchen extension quote can become once all costs are included:

Item	Cost
Builder's quote inc VAT	£60,000
Architect and structural engineer	£2,500
Planning / Lawful Development Certificate	£500
Building control fees	£900
Skips	£1,200
Scaffolding	£1,000
Kitchen units, worktops and appliances	£18,000
Flooring, tiling and decoration	£6,000
Contingency 15%	£13,665
Total realistic budget	£103,765

That is a significant step up from the headline figure. The homeowners who avoid stress are the ones who plan for the full number, not just the builder's quote.

How to keep costs under control

1. Fix the design before quoting. Changes during the build are expensive.
2. Get three itemised quotes. Compare the same scope and specification.
3. Lock in material prices. Ask your merchant for a written quote.
4. Set a realistic contingency. 15% is sensible for most projects.
5. Choose finishes early. Last-minute kitchen or tile decisions cause delays.
6. Avoid scope creep. Every extra door, window and socket adds labour and cost.

For advice on comparing builder quotes, read our guide to how to compare builder quotes line by line.

Financing an extension

Most homeowners fund extensions through savings, remortgaging or a further advance on their mortgage. Some use a personal loan for smaller projects. Whichever route you choose, have the full budget — including fees, finishes and contingency — approved before work starts. Running out of money halfway through is one of the most stressful situations in construction.

Will an extension add value?

A well-designed extension usually adds value, but not always as much as it costs. The best returns come from adding usable living space that matches what buyers in your area want. A kitchen-diner extension in a family area is likely to add more value than a small home office in a location where space is already tight.

Think about value, but do not build purely for resale. You are the one who will live with the space.

Final thoughts

Extension costs are not a mystery, but they are wider than a single headline figure. Build your budget from the ground up: construction, professional fees, statutory fees, finishes and contingency. Get written quotes, compare them fairly and plan for the realistic total, not the best-case number.

If you are planning an extension in the South East, request a materials quote from Star Hawk Builders Merchant. We can also help you find your nearest branch and you can browse our case studies to see the kind of projects we support.