House Extension Cost Calculator
Our House Extension Cost Calculator helps you understand realistic extension costs in the UK based on current construction rates. Whether you are planning a single storey extension, double storey addition, loft conversion or wrap-around extension, this house extension cost guide provides typical build cost ranges per square metre to help you budget properly from the outset.
However, while this extension cost calculator offers a reliable starting point, the only way to obtain accurate extension costs is through a fully detailed tender pack prepared by a professional architectural team and issued to builders for competitive pricing before any commitment is made.
House Extension Cost Calculator
Construction costs vary depending on the type of work. New floor area is calculated at a higher rate per m², while upper-floor additions are typically cheaper because groundworks and slabs are reduced. Existing internal refurbishment is calculated separately at a lower rate per m². Bathrooms, en-suites, WCs and the kitchen are priced separately below to avoid double counting. Results update instantly.
The cost ranges used in this calculator are based on construction pricing from recently completed residential projects across the UK.
For accurate pricing, the best approach is a fully detailed tender package issued to multiple builders before any commitment is made.
How this house extension cost calculator works
Our house extension cost calculator provides a realistic starting point for estimating construction costs in the UK. By adjusting the floor area, specification level and internal refurbishment scope, you can quickly see how project size and quality affect the overall budget. The calculator uses typical build rates per square metre and separates items such as bathrooms and kitchens to avoid double counting. While every project is different, this tool helps homeowners understand the likely cost range before speaking to a professional architectural team.
Typical house extension costs in the UK
House extension costs in the UK vary significantly depending on specification, structural complexity and site constraints. A simple single-storey extension with standard finishes will usually cost less per square metre than a high-specification design with large glazing, bespoke joinery or structural alterations. Upper floor additions are often slightly cheaper than new ground floor construction because foundations and slabs are already in place. However, structural work, steel beams and roof alterations can still add significant cost.
Why tender drawings are essential for accurate pricing
Online calculators are useful for early budgeting, but the only reliable way to obtain accurate construction prices is through a detailed tender package. Professional architectural drawings allow multiple builders to price the same information, making it possible to compare quotations properly. Without this level of detail, builders are often forced to estimate or include large risk allowances, which can result in inflated costs or unexpected variations later in the project.
Planning a house extension? Get a realistic cost before you commit.
If you’re planning a house extension, we can review your project and help you understand realistic construction costs before speaking to builders.
Carter Designs has over 28 years of experience and is a Chartered Institute of Architectural Technologists practice working across Kent, London and Harrogate.