How Much Does a House Extension Cost in London in 2025?

If you're considering extending your home in London, one of the first questions you're likely to ask is:
"How much is this going to cost?"

It's a good question—and an important one. While every home and project is different, there are clear cost ranges you can use to start budgeting with confidence. This article breaks down the key factors that affect cost, typical price bands for different types of extensions, and what else you need to budget for beyond the build.

Why Extension Costs Vary

The cost of a home extension in London depends on several factors, including:

• The type and size of the extension (rear, side return, wraparound, loft, etc.)

• Your property's structure, age, and access conditions

• Site complexity (e.g. sloping ground, shared party walls)

• Whether you're in a conservation area or listed building

• The quality of finishes, glazing, and bespoke elements

• Planning permission requirements and potential delays

A 3-metre rear extension with standard finishes will cost significantly less than a highly glazed wraparound extension with structural complexity and high-end detailing.

For a modest 4-metre rear extension of 20m², the build alone might range from £150,000 to £200,000+vat, depending on specification.

Important: Always allow for an additional 20–25% on top of the build cost to cover design, professional fees, approvals, and contingency.

What Else to Budget For

It’s common for homeowners to underestimate the additional costs involved in a home extension project. These often include:

• Architectural design fees

• Planning application fees (typically £250–£400)

• Structural engineer and drainage consultant fees

• Building regulations drawings and inspections

• Party wall surveyor (if applicable)

• Interior fit-out costs (kitchens, flooring, lighting)

• VAT (unless your project qualifies for zero or reduced rate)

• Contingency fund (usually 10–15%)

For most of our clients, the all-in cost of a well-designed and well-built extension in London ranges from £150,000 to £350,000, depending on size, quality, and complexity.

Case Study: A Rear and Side Return Extension in London

We recently worked with a young family in Walthamstow who needed more space for cooking, dining, and working from home. We designed a 3.5m rear and side return extension to their Victorian terrace with large-format glazing, timber flooring, and bespoke cabinetry.

• Build cost: approx. £210,000

• Total project cost (inc. fees and finishes): approx. £250,000

• Planning outcome: Approved on first submission

The result was a light-filled, future-proofed family space with direct access to the garden and a much-improved layout for everyday living.

Frequently Asked Questions

Can I build an extension for under £100k?
Yes, if it’s a smaller or simpler build with standard finishes. For more ambitious designs or larger spaces, most projects in London tend to exceed this.

Do I need planning permission?
Many rear extensions fall under permitted development rights, but not all. If you're in a conservation area or planning something over a certain size, full planning permission is likely required.

Can I live in the house during the works?
Sometimes. It depends on the nature of the works, your tolerance for disruption, and the sequence of construction. We’ll advise on this during the early design stages.

Ready to Take the Next Step?

If you're thinking about extending your home and want clear advice on what’s possible within your budget, we offer a free 30-minute online consultation.

You’ll get honest insight into feasibility, planning, and what it will take to bring your ideas to life.