- What Is a Single-Storey Rear Extension?
- How Much Does a Single-Storey Rear Extension Cost in London in 2026?
- Planning Permission: What You Need to Know
- Design Choices That Make or Break a Rear Extension
- The Build Process: What to Expect
- Managing Your Budget Without Surprises
- How MVV Approaches Single-Storey Rear Extensions in London
- Frequently Asked Questions
A single-storey rear extension is one of the most popular ways London homeowners add space without moving. Done well, it opens your kitchen to the garden, creates a proper family room, and adds real value to your home. Done badly, it drains your budget and causes months of unnecessary stress.
This guide covers what a single-storey rear extension actually costs in London in 2026, which planning rules apply, which design decisions matter most, and how to keep the whole process under control.
What Is a Single-Storey Rear Extension?
A single-storey rear extension is a ground-floor addition built onto the back of your house, extending your footprint into the garden and connecting to your existing kitchen, dining room, or living space.
Most London homeowners use them to create an open-plan kitchen-diner, a larger family room, or a dedicated workspace. The extension can run the full width of the rear elevation or just part of it, depending on your garden depth and what planning allows.
How Much Does a Single-Storey Rear Extension Cost in London in 2026?
Cost is almost always the first question, and the honest answer is: it depends on size, specification, and location. London build costs sit noticeably higher than the national average, driven by labour rates, access constraints, and the realities of working in a dense urban environment.
As a general guide for 2026:
| Extension size | Basic specification | Mid-range specification | High specification |
|---|---|---|---|
| 15 sqm | £45,000 – £55,000 | £55,000 – £70,000 | £70,000 – £90,000+ |
| 25 sqm | £65,000 – £80,000 | £80,000 – £105,000 | £105,000 – £135,000+ |
| 40 sqm | £95,000 – £120,000 | £120,000 – £155,000 | £155,000 – £200,000+ |
These ranges cover structural work, roofing, glazing, internal finishes, and connection to your existing space. Architect fees, planning applications, and furniture are not included.
What Drives the Cost Up
Several factors push a London rear extension towards the higher end of any range:
- Basement or poor ground conditions requiring deeper or more complex foundations
- Flat roof with lantern or rooflight rather than a simple pitched or lean-to roof
- Full-width bifold or sliding doors across the rear elevation
- Underfloor heating instead of standard radiators
- Bespoke joinery such as fitted cabinetry or a custom kitchen
- Party wall agreements with both neighbours, which add surveyor fees and sometimes structural costs
- Listed building status or conservation area restrictions requiring specialist materials or additional consents
What Keeps Costs Manageable
A clear brief from the start saves money. Every change made after work begins costs more than the same decision made during design. Choosing a team that actively tracks your budget throughout the project — not just at the quote stage — also makes a real difference.
At MVV, monthly progress reports include budget tracking and on-site photography, so you always know where your money is going. No surprises at the end.
Planning Permission: What You Need to Know
Many single-storey rear extensions in London fall under Permitted Development (PD) rights, meaning no full planning application is needed. That said, London has specific rules that catch people out more often than you might expect.
Permitted Development Rules for Rear Extensions in 2026
Under current PD rules, a single-storey rear extension can extend:
- Up to 4 metres beyond the original rear wall for a detached house
- Up to 3 metres for a semi-detached or terraced house
The extension must not exceed 4 metres in height, must not cover more than half the garden area, and must use materials that match the existing house in appearance.
When You Will Need Full Planning Permission
A full planning application is required if:
- Your property is in a conservation area, Article 4 Direction zone, or is a listed building
- The extension exceeds the PD size limits
- You want to extend beyond the side elevation
- Your local council has removed PD rights through an Article 4 Direction
A large portion of inner London sits within conservation areas or Article 4 zones. If you are in Chelsea, Notting Hill, Islington, or similar areas, check your local planning authority's interactive map before assuming PD applies.
The Neighbour Consultation Scheme
If your extension falls between 3 and 6 metres (semi-detached or terraced) or between 4 and 8 metres (detached), you can still build under PD but must go through the Neighbour Consultation Scheme. Your council notifies adjoining owners, who have 21 days to raise concerns. If they do, the council decides whether to approve the larger extension.
Party Wall Agreements
Any extension that builds up to or along a shared boundary — or involves excavation within 3 to 6 metres of a neighbour's foundation — triggers the Party Wall Act. You will need a party wall surveyor and a formal agreement in place before work starts. Budget around £800 to £1,500 per neighbour for a straightforward agreement.
Design Choices That Make or Break a Rear Extension
The structural decisions are relatively straightforward. The design decisions are where you shape how the space actually feels to live in.
Roof Type
Flat roof with internal lantern or rooflight: The most popular choice for modern open-plan extensions. A large rooflight floods the space with natural light and keeps the visual profile low. Quality matters here — cheap rooflights leak.
Lean-to (mono-pitch) roof: Slopes away from the house. Simpler and often cheaper. Works well on narrower extensions and suits both period and contemporary properties.
Pitched roof: Matches the existing house more closely, which can help in conservation areas. Slightly more expensive to build but can add real character.
Rear Glazing
The rear wall is where most homeowners concentrate their design budget. Your main options are:
- Bifold doors: Open the full width of the extension. Popular, but they need clear floor space when folded back.
- Sliding doors: Cleaner sightlines, better suited to tighter spaces. Typically two or three panels.
- Crittal-style steel framing: A strong aesthetic choice for period properties. Higher cost, but distinctively good-looking.
- Full-width fixed glazing with a separate door: Maximises the view without the mechanical complexity of a folding system.
Internal Layout
Before you finalise the structural opening, think carefully about how the new space connects to your existing ground floor. A wider opening creates a genuinely open-plan feel. A narrower one can end up feeling like a corridor. Your designer should walk you through this with drawings before any structural work begins.
The Build Process: What to Expect
A single-storey rear extension in London typically takes 12 to 20 weeks on site, depending on size and specification. The design and planning phase adds time before that.
A realistic timeline looks like this:
- Initial design and cost estimate (2 to 4 weeks)
- Planning application or PD confirmation (8 to 10 weeks if planning is needed)
- Technical drawings and structural engineering (3 to 4 weeks)
- Party wall agreements (run in parallel with planning where possible)
- Build phase (12 to 20 weeks on site)
- Snagging and handover (1 to 2 weeks)
The biggest delays come from planning, party wall disputes, and late design changes. Getting your brief right early — and working with a team that manages the process actively — keeps the timeline on track.
Managing Your Budget Without Surprises
Budget overruns are the fear most homeowners raise before starting a project. They tend to happen for three reasons: an incomplete initial specification, unforeseen structural issues, and changes made during the build.
You can reduce the risk significantly by:
- Agreeing a detailed specification before work starts, not just a headline price
- Building a contingency of 10 to 15 per cent into your budget for unforeseen issues
- Choosing a contractor who tracks spend against budget monthly, rather than only at the start and end
Monthly budget reports with on-site photography are not standard practice across the industry. When a contractor offers them as a named feature, it signals genuine financial transparency throughout the project — not just at the quote stage.
How MVV Approaches Single-Storey Rear Extensions in London
MVV handles single-storey rear extensions from concept to completion under one team. Design, planning, project management, and build all sit together, which means no gaps between what the architect draws and what the builder delivers.
If you want a starting point before any conversation, the instant cost estimator at themvv.co.uk gives you a ballpark figure in minutes, with no commitment required. It is a useful first step before you speak to anyone.
MVV has completed projects across London, including Chelsea SW3 and Ladbroke Grove, working with homeowners who want more space and more certainty from their build.
Frequently Asked Questions
How much does a single-storey rear extension cost in London in 2026?
Costs vary by size and specification. A 15 sqm extension at mid-range specification typically costs between £55,000 and £70,000 in London. A 25 sqm extension at the same specification runs from £80,000 to £105,000. High-specification finishes, complex glazing, and poor ground conditions all push costs higher.
Do I need planning permission for a single-storey rear extension in London?
Many rear extensions fall under Permitted Development rights and do not need a full planning application. However, conservation areas, Article 4 Directions, and extensions beyond the PD size limits all require full planning permission. Always check with your local planning authority before assuming PD applies — particularly in inner London boroughs.
How long does a single-storey rear extension take to build?
The on-site build phase typically takes 12 to 20 weeks. Factor in design, planning, and party wall stages and the full process from initial brief to handover is usually 6 to 12 months. Planning permission, where required, is often the longest single step.
Do I need a party wall agreement for a rear extension?
You will need one if your extension builds along a shared boundary or involves excavation within 3 to 6 metres of a neighbour's foundation. A party wall surveyor handles the process and typically charges £800 to £1,500 per neighbour for a straightforward agreement.
What is the best roof type for a single-storey rear extension?
A flat roof with a large rooflight or internal lantern is the most popular choice for modern open-plan extensions in London. It maximises natural light and keeps the visual profile low. A lean-to roof is simpler and often cheaper. A pitched roof suits properties where matching the existing roofline matters, such as in conservation areas.
How can I avoid budget overruns on my extension?
Agree a detailed specification before work starts, build a 10 to 15 per cent contingency into your budget, and choose a contractor who provides monthly budget reports throughout the project. Late design changes are the most controllable cause of overspend.
Can I build a single-storey rear extension on a terraced house in London?
Yes. Terraced houses are among the most commonly extended properties in London. The PD allowance is 3 metres beyond the original rear wall, extendable to 6 metres under the Neighbour Consultation Scheme. Party wall agreements with both neighbours are typically required. Many terraced houses in inner London also sit within conservation areas, which affects both materials and design.
A single-storey rear extension is a well-tested way to add space, improve your daily life, and increase the value of your London home. The key is getting the design right, understanding the planning rules that apply to your specific property, and working with a team that keeps your budget visible from start to finish.
To find out what your project might cost before any conversation, get a free instant estimate at themvv.co.uk.