Extension Cost Calculator

House Extension Costs Calculator

The current uncertainty in the housing market has caused an upsurge in the number of house extensions being built. Adding a bedroom to a house is an almost certain way of increasing the value of a property beyond the cost of having the work done.

Whatprice provide a useful cost calculator (below), or better still why not use our FREE quotation service to get an accurate cost for building an extension.

With the extension cost calculator you can get a quick, free, rough estimate of at least some of the costs involved in building extra room(s) for your house. By entering information about the extensions external size, roof type, number of storeys and the UK location, an estimate of the building cost can be found. The location of the build is needed, as costs do vary across the country and the price is weighted according to this information.

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Extension Diagram


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About the Extension Cost Calculator

The online extension calculator displayed above produces approximate costs of various sizes and types of extensions plus also weights it based upon the buildings location.

The data is based upon input from a quantity surveyor on real home extensions ranging in size from 2x3m to 4x6m. Extensions outside of these sizes are likely to be less accurate.

These extension costs exclude:

It should be remembered that the extension building costs displayed are indicative only and quotations should be obtained from reputable contractors - Disclaimer.

The following table compares the estimate from the extension cost calculator with actual prices submitted to Whatprice. Remember the calculator does not include kitchen or bathroom fittings and is only meant to be a guide.

Compare Calculator Estimates
Detailed Description of WorkActual PriceCalculated Cost
Single storey kitchen/dining room extension 25m2 in total including fitting of kitchen 2 radiators, move boiler, 12 spot lights, bi fold screen doors, 3 velux windows full decoration, all planning, etc taken care of. Had 4 quotes ranging from 30000 to 48000 went with the cheapest quote. £30,000£30,826
3m x 3m one storey to rear to make bigger kitchen. All building work including roof, wiring, plumbing, plastering and new kitchen. Very happy with finished addition to my home. £18,000£14,672
Single storey extension to bungalow. External dimensions 4m by 7m approx. All works included except decoration and provision of bathroom fittings although plumbing and fitting of sanitary ware included. Pitched and tiled roof. To provide bedroom and bathroom. Some works needed to existing building to provide entrance to new part - Such as repositioning window and radiator, blocking up 2 windows not now needed. New Electric consumer unit as required. Price includes VAT. Plans drawings and building control cost extra ( about 1000). £18,000£14,672
Double storey 6.6m x 4.1m, downstairs toilet/shower room , small utillity, large dining room, upstairs bedrooms, knock through upstairs to continue hallway, plasterboarded, plaster finish, all electrics, plumbing included, wayrock flooring to first floor, pitched roof with gable. £48,000£44,686