a CarlArchitect-designed single-storey extension

How much will my house extension cost?

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Let us help you estimate the cost of your extension cost or renovation

Estimating a budget for work on your house is extremely difficult. The cost of a house extension will usually come down to the scale of the project. But other factors include:
  • the extent of site preparation and groundwork required.
  • the quality of internal fixtures and fittings you want.
  • where you live in the country.
It’s especially tricky in the current climate where manufacturing/supply and demand issues are causing price fluctuations on many materials. The quotes you obtain from builders will also vary, but beware of going for the cheapest quote. A good builder will not cut corners and you’re less likely to run into problems that cost you money down the line. A builder’s quote may not be the total bill for your extension. Check what the quote includes and bear in mind the costs of fixtures and fittings. You may also need planning permission, building regulations approval and, for major structural changes, the services of a structural engineer.

Cost estimates for your building work

Here we are providing an overview and estimated price range for how much your house extension or refurbishment cost may be based on our experience. These figures should be used as guidance only, are based on Spring 2023 prices and exclude VAT. A single-storey extension

New single-storey extension

£2,200 – £3000 per m2 additional floor area The lower range would be a basic construction with uPVC windows/doors. The higher range would be a more complex construction with specialist glazing. Note that small extensions like side-return extensions may have a much higher £/m2 rate because of complexity.

New additional storey extension

£1,500 – £1,750 per m2 additional floor area The cost is less than a single-story extension because when building a second storey, you no longer have the groundwork costs, which can be considerable.

Major structural openings (steel frame)

steel frame opening £3,000 – £10,000 per opening. When creating an open-plan kitchen/living area, for example, this cost would be relevant to removing the house’s existing ground floor rear wall.

Loft Conversion

£2,000 – £2,700 per m2 of additional floor space (above 1500mm ceiling height) The cost of this will rise with the complexity of your existing roof structure. If your roof and proposed layout are complex, it may be worth considering removing the existing structure completely and installing a new roof.
Basement under existing house plus underpinning
basement conversion £3,000 – £5,000 per m2 additional floor space. The wide range in cost reflects site accessibility and logistics.

Basement under a garden

£3,000 – £4,000 per m2 additional floor space This can be a great option in conjunction with walk-on rooflights to let light into your new basement space.

Kitchensbased on 20 kitchen units and including appliances 

Victorian Garden Room Extension
  • Basic: £6,000+ e.g., Howdens/Magnet, basic appliance spec, laminate worktops.
  • Mid-range: £15,000+ e.g., Wren/IKEA, better appliance spec (e.g., siemens), stone worktops
  • High spec: £40,000+ bespoke/designer manufacturer, top appliance range (e.g., Miele), stone worktops.

Internal refurbishment including minor structural works

£1000-£1500 per m2 refurbished floor area, including minor structural work, layout changes, new wiring/plumbing, taking the interior back to brick. £500-£750 per m2 for just cosmetic changes only.

Removing chimneys

£2,000 – £5,000 per chimney. Depending on height, new structural details, site access, ownership and type of construction.

New 3-piece bathroom fully tiled

Including carpentry, plumbing and electrics shell sanitary ware and tiles and depending on the level of finish:
  • Basic £2,500+
  • Mid-range £5,000 +
  • High spec £10,000+
  The information contained in this article is for general informational purposes only, does not constitute professional advice and does not necessarily reflect the official policy or position of any company or organisation. Readers should always seek professional advice before undertaking any action based on the information contained in this article. The author makes no representations or warranties of any kind, express or implied, about the completeness, accuracy, reliability, or suitability of the information, products, services, or related graphics contained in the article for any purpose. 

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