How Much Does an Extension Cost in NW London?

Building an extension is one of the most popular ways to add space and value to a home in NW London. From single-storey rear extensions in Belsize Park terraces to double-storey side returns in Hampstead, the scope — and cost — can vary enormously. This guide provides indicative cost estimates to help you understand what to expect.

All figures quoted are indicative estimates and should not be treated as quotations. We are an information and matching service and do not carry out building work. Always obtain detailed quotes from qualified contractors for your specific project.

Indicative Extension Costs

The cost of an extension depends on its size, specification, structural complexity, and the constraints of your site. Below are broad indicative ranges for NW London:

On a per-square-metre basis, single-storey extensions in the Hampstead area typically fall in the range of £2,200–£3,500 per m², depending on specification. For a more detailed breakdown, see our cost per square metre guide.

Key Cost Factors

Foundations and Ground Conditions

NW London sits on London Clay, which can present challenges. If nearby trees require deeper foundations, or if the water table is high, your groundwork costs will increase. Expect foundation costs alone to range from £5,000–£15,000 depending on depth and complexity.

Structural Requirements

Opening up the rear wall of a Victorian or Edwardian terrace typically requires a steel beam (RSJ), which adds structural engineering costs. Our structural engineer costs guide explains what to budget for this essential professional.

Specification and Finishes

The single biggest variable in extension costs is the specification. A basic extension with standard finishes will sit at the lower end of the range. Bi-fold doors, underfloor heating, high-end kitchen installations, and bespoke joinery can push costs significantly higher. See our kitchen renovation cost guide if your extension includes a new kitchen.

Conservation Area Considerations

Large parts of Hampstead, Belsize Park, and Highgate fall within conservation areas. This can affect your choice of materials — for example, requiring natural slate roofing rather than concrete tiles, or matching brickwork rather than render. These requirements add cost. Our conservation area renovation costs guide explains the premium in detail.

Professional Fees

In addition to the build cost, budget for:

• Architect fees — typically 7–12% of construction cost for an extension project. A good architect will maximise the value of your space while navigating planning constraints. Architect Hampstead can connect you with experienced local practices.
• Structural engineer — £1,500–£3,500 for beam calculations and foundation design.
• Planning application — if required, the statutory fee for a householder application is currently £258. Many single-storey rear extensions fall under permitted development, but conservation area restrictions may still apply. Planning Hampstead provides expert guidance on whether your project needs planning permission.
• Building regulations — inspection fees of £500–£1,200 depending on the scope.
• Party wall agreements — if your extension is within 3 metres of a neighbouring property (or 6 metres for deeper foundations), you will need party wall agreements. Budget £1,000–£1,500 per neighbour.

Timeline

A typical single-storey extension in NW London takes 12–16 weeks on site, with 8–12 weeks of design and approvals beforehand. Our renovation timeline guide provides a detailed breakdown of each phase.

Maximising Value

To get the most from your extension budget, consider engaging an architect early to explore options and avoid costly changes later. Value engineering — reviewing the design to find savings without compromising quality — can typically save 10–20% on construction costs. Read our value engineering guide for practical strategies.

This content is provided for general information purposes only. Costs are indicative and based on market data at the time of publication. Always obtain professional advice and detailed quotations specific to your project.