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Extension Cost Calculator

Get an accurate estimate for your house extension project. Our calculator takes into account your property type, location, and specific requirements.

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Property

Tell us about your property

This helps us provide a more accurate cost estimate for your extension

What type of property do you have?

Terraced House

Connected to neighbouring properties

Semi-Detached House

Connected to one neighbouring property

Detached House

Standalone property

Flat/Apartment

Part of a larger building

Maisonette

Two-storey flat

Where is your property located?

Zone 1

Central London

Zone 2

Inner London

Zone 3

Outer London

Zone 4

Greater London

Zone 5+

Beyond Greater London

Extension Diagram

Select extension type to see diagram

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Quick Tips

  • • Measure your extension area accurately
  • • Consider planning permission requirements
  • • Factor in additional features you want
  • • Get multiple quotes for comparison

FAQ

Frequently Asked Questions

  • House extension costs in the UK typically range from £1,500 to £3,000 per square metre, depending on the type of extension, materials used, and location. A single-storey extension usually costs between £1,500-£2,500 per m², while a double-storey extension ranges from £2,000-£3,000 per m².

    Our calculator provides accurate estimates based on your specific requirements, property type, and location to help you budget effectively for your project.

  • Many house extensions can be built under permitted development rights, which don't require planning permission. Single-storey rear extensions up to 4m for detached houses or 3m for other properties are usually permitted.

    However, you'll need planning permission for larger extensions, those in conservation areas, or if your property has already been extended. Always check with your local planning authority before starting work.

  • The timeline for building a house extension varies depending on size and complexity. A simple single-storey extension typically takes 8-12 weeks, while a double-storey extension can take 12-20 weeks.

    Complex extensions with additional features may take 6-8 months. Factors affecting timeline include weather conditions, material availability, and any planning permission delays.

  • The most popular house extension types include rear extensions (most common), side extensions, wrap-around extensions, and loft conversions. Rear extensions are favoured for creating open-plan living spaces and kitchen extensions.

    Side extensions are ideal for adding extra bedrooms or expanding existing rooms. The choice depends on your property layout, available space, and intended use.

  • There are several ways to finance a house extension: remortgaging your property, taking out a home improvement loan, using savings, or applying for a construction mortgage.

    Remortgaging is often the most cost-effective option as it typically offers lower interest rates. Consider the total cost including VAT, professional fees, and contingency funds when planning your budget.

  • When designing your house extension, consider: the intended use and functionality, natural light and ventilation, how it connects to existing rooms, building regulations compliance, energy efficiency, and future resale value.

    Work with an architect or designer to create a space that enhances your lifestyle while adding value to your property. Consider the flow between old and new spaces.

  • Yes, all house extensions must comply with building regulations covering structural integrity, fire safety, energy efficiency, ventilation, and accessibility.

    You'll need building control approval, which involves inspections at key stages of construction. Your builder or architect should handle the building control process, but it's important to ensure all work meets current standards.

  • A well-designed house extension can add significant value to your property, typically 10-20% of your home's current value. Kitchen extensions often provide the best return on investment, followed by additional bedrooms and living space.

    The exact value increase depends on your location, the quality of the build, and how well the extension integrates with your existing property.

  • Common challenges include: managing the construction process while living in the property, dealing with unexpected structural issues, coordinating with contractors and suppliers, managing noise and disruption, and staying within budget.

    Good planning, clear communication with your builder, and having a realistic timeline can help minimise these challenges.

  • While not always legally required, hiring an architect is highly recommended for house extensions. They can help with design optimisation, planning permission applications, building regulations compliance, and contractor coordination.

    Architects can also help maximise your space and ensure the extension adds value to your property. For complex projects or if you want a unique design, an architect is essential.