Single Storey Extension Cost London: 2026 Price Guide

A single storey rear extension is the most popular home improvement in London — and for good reason. It's the most straightforward way to add meaningful space to a terraced or semi-detached house without the planning complexity of a double storey. But costs vary significantly depending on size, specification, and where you are in London. Here's what you need to know in 2026.

Single Storey Extension Costs in London at a Glance (2026)

Cost per square metre

Single storey extensions in London cost between £3,000 and £5,000+ per square metre for the build, before professional fees, fit-out, or VAT. The variation reflects the enormous range in specification — from a basic flat-roof addition to a full-width glazed kitchen extension with a roof lantern and bi-fold doors.

Typical total project costs

• 20m² at mid-range spec: £80,000–£120,000 all-in

• 25m² at mid-range spec: £100,000–£140,000 all-in

• 30m² at mid-range spec: £110,000–£165,000 all-in

• 30m² at high spec with premium kitchen: £160,000–£220,000+

These figures include construction, professional fees (architect, structural engineer, planning, building regulations), and VAT. They exclude fit-out beyond basic plastered walls and screed floor.

What Affects the Cost of a Single Storey Extension?

Size

Larger extensions cost more in absolute terms, though cost per square metre can reduce slightly at larger sizes due to economies of scale in foundations, roofing, and scaffolding.

Specification level

This is the biggest variable. The difference between a basic flat-roof extension and a high-specification glazed extension with a roof lantern, bi-fold doors, underfloor heating, and a premium kitchen fit-out can easily be £70,000 or more on the same footprint.

Location within London

Labour rates and logistics costs vary significantly across London's 32 boroughs. Inner London (Zones 1–2) commands the highest rates. Outer London boroughs can be 20–25% cheaper for the same specification.

Whether planning permission is needed

Many single storey rear extensions fall under permitted development and don't need a planning application, saving you the application fee and several months of time. However, if your home is in a conservation area, or if your proposed extension exceeds the permitted development limits, you'll need full planning permission — adding cost and time to the project.

Basic, Mid-Range, and High-Spec: What Do You Actually Get?

Basic specification: brick or blockwork construction with a flat roof, standard UPVC double-glazed doors and windows, insulated to current building regulations minimum, plasterboard walls and screed floor ready for decoration. This is the most affordable route and can look excellent with good design.

Mid-range specification: the same solid structure but with better quality glazing — aluminium bi-fold or sliding doors, a roof lantern or rooflight, and a higher quality of internal finish. This is the most common choice for London kitchen extensions and typically what you'd expect from a quality residential contractor.

High specification: premium aluminium glazing systems, structural glass, a fully specified kitchen (often £25,000–£50,000+ for units and appliances alone), underfloor heating, polished concrete or large-format tile flooring, bespoke joinery. The build quality and finish here is genuinely impressive — and priced accordingly.

Full Cost Breakdown: Where Your Money Goes

• Construction (shell only): £55,000–£110,000 for a 20–30m² extension

• Architect / design and planning consultant: £4,000–£12,000

• Structural engineer: £1,500–£3,000

• Planning application fee: £548 (from April 2026)

• Building regulations: £800–£1,500

• Kitchen fit-out: £15,000–£50,000+ (additional)

• Flooring and decoration: £3,000–£15,000 (additional)

The most common budgeting mistake is comparing a build-only quote from a contractor with an all-in project cost from a design and planning consultant. Make sure you're comparing like with like by understanding exactly what each quote includes.

Planning and Permitted Development for Single Storey Extensions in London

What you can build without planning permission

Under permitted development rights, a single storey rear extension on a house (not a flat) can extend up to 3 metres beyond the original rear wall for terraced and semi-detached properties, and up to 4 metres for detached houses — without needing a planning application. The extension must be no higher than 4 metres, and cannot be taller than the existing eaves.

The Larger Home Extension scheme

Under the prior approval process, you can extend up to 6 metres (terraced/semi) or 8 metres (detached) beyond the original rear wall without full planning permission, subject to prior approval from your council and no objections from neighbours within the required notification period.

When you need full planning permission

You'll need a full planning application if your home is in a conservation area, if the extension exceeds permitted development limits, if you're extending to the side rather than just the rear, or if your property has had permitted development rights removed by an Article 4 Direction. At Studio : MASS, we check your permitted development position at the very first stage of our process — before you commit to any design work.

Single Storey vs Double Storey — Which Gives Better Value?

If you primarily need more ground floor space — a larger kitchen, an open-plan kitchen-diner, or a playroom — a single storey extension is almost always the right choice. It's faster to build, less likely to need planning permission, and causes less disruption during construction. If you also need more bedrooms or bathrooms, a double storey extension adds far more space per pound spent than two separate projects.

How Long Does a Single Storey Extension Take in London?

• Feasibility and design: 4–6 weeks

• Planning application (if required): 8–10 weeks

• Building regulations drawings: 4–6 weeks (can run in parallel with planning)

• Contractor procurement: 4–8 weeks

• Construction on site: 10–16 weeks

• Total from first call to completion: typically 6–10 months

The timeline varies depending on whether planning permission is required and how smoothly the contractor procurement stage goes. At Studio : MASS, we map out a realistic programme for every project from day one so you're never left guessing where you are in the process.

How to Get Accurate Quotes and Compare Builders

Wait until you have detailed architectural drawings and a schedule of works before inviting contractors to price. Early ballpark figures based on square meterage alone are unreliable and often lead to significant variations once the true scope becomes clear. A good quote should specify exactly what materials are being used, what's included in the build, and what's been excluded. Any quote that comes in significantly lower than others should be questioned carefully — in London's competitive contractor market, there's usually a reason.

Frequently Asked Questions

Does a single storey extension always need planning permission?

No — many single storey rear extensions on houses fall under permitted development rights and don't require a planning application. However, this depends on the size, your property type, and whether you're in a conservation area. Studio : MASS checks your position at the feasibility stage.

How long does a single storey extension take to build?

The construction phase typically takes 10–16 weeks for a standard single storey extension. The full project timeline from appointing a design consultant to completion is usually 6–10 months.

Can a single storey extension be done for £60,000?

At basic specification and around 15–18m², it's possible in outer London boroughs. In inner London or at mid-range specification, £60,000 is unlikely to cover a complete, quality project. Always seek a detailed quote against a proper specification rather than a rough per-square-metre estimate.

Thinking about a single storey extension? Book your free architectural advice call with Studio : MASS. We'll assess your property, confirm your permitted development position, and give you an honest picture of what's achievable within your budget.