A single-storey extension is a new ground-floor room built onto your house, usually at the rear or side, with its own foundations, walls and roof, creating a bigger kitchen, living space or bedroom without moving home.
For a homeowner in Chelsea, planning a single-storey extension tends to begin with two questions: what will it cost, and how disruptive will it be? Our guide range is £45,500–£98,000 including VAT, and a typical job runs to 10–16 weeks on site. Both numbers firm up once we have seen the property, measured up and agreed exactly what is included.
- Guide price
- £45,500–£98,000 inc VAT
- Time on site
- 10–16 weeks on site
- Postcodes
- SW3, SW5, SW7, SW10 and SW1W
- Planning rules
- England
- Local area
- Greater London
- Survey
- Free, with a written quote
What should you budget for a single-storey extension in Chelsea?
Budgeting for a single-storey extension in Chelsea? The numbers below include VAT and are adjusted for London, where costs are about 30% above the national guide. The headline range is £45,500–£98,000. Nothing is final until we have measured up at a free survey and agreed the specification with you.
| Type of job | What it usually involves | Guide in Chelsea |
|---|---|---|
| Small extension (8–12m²) | A modest rear or side extension, such as a utility room or a slightly larger kitchen. | £28,500–£52,000 |
| Typical rear extension (15–25m²) | A kitchen-diner or family room with bifold doors and a rooflight or lantern. | £45,500–£98,000 |
| Large or wrap-around (30m² plus) | A wrap-around or wide rear extension, often with structural steelwork into the house. | £91,000–£169,000 |
Your price depends mostly on:
- Floor area, shape and number of corners
- Ground conditions, foundation depth and drains to divert
- Steelwork needed to open into the existing house
- Finishes, kitchen, flooring and heating specification
What’s included?
- Groundworks, foundations and below-ground drainage
- Cavity walls built and insulated to building regulations
- Flat or pitched roof, with rooflights or a lantern if specified
- Steel beams to open into the existing house
- Electrics and plumbing by qualified trades, with certificates
- Windows and external doors, including bifolds if chosen
Building in Chelsea
London's housing is dominated by Victorian and Edwardian terraces, often in yellow London stock brick, with Georgian terraces in central districts and mansion blocks across many inner areas. Outer boroughs have large belts of interwar semis, and there are extensive post-war council estates. Many houses have been split into flats, and conservation areas are widespread.
London Clay shrinks and swells with the seasons, and street trees close to houses can contribute to movement, which affects foundation design.
That variety is why a guide price for a single-storey extension in Chelsea is only a starting point; the survey shows what your particular property needs.
Do you need permission in Chelsea?
As Chelsea is in England, read the following with that system of planning and building control in mind. We check what applies to your house with the council before you receive a quote.
In England, many single-storey rear extensions are permitted development: typically up to 3 m deep on a semi-detached or terraced house and 4 m on a detached house, no more than 4 m high, and covering no more than half the land around the original house. Larger rear extensions can go through the prior approval process. Side extensions, flats, listed buildings, conservation areas and homes where permitted development rights have been removed need planning permission.
Building Regulations approval is needed for almost every extension, covering foundations, structure, insulation, drainage, ventilation and fire safety. If the work is on or near a shared wall or boundary, the Party Wall etc. Act 1996 may apply.
Rules change and every property is different, so we check the planning history and constraints for your address before we quote.
How does the job run?
We look at the job
It starts with a survey at the property, free of charge. We check condition, access and services, ask how you want the finished result to look, and answer any questions you have about a single-storey extension.
Written quote
After the survey we send a quote for a single-storey extension that fixes the price for the scope we've agreed. If you want to change materials or scope, we revise it in writing before anything is booked.
Getting ready
We handle the paperwork side: checking whether planning permission or building control approval is needed, submitting applications where they are, and scheduling the trades and materials around your dates.
Build and hand over
Our trades complete a single-storey extension while your point of contact keeps you updated. When everything is finished, we clear the site, take away waste and go through the result with you before handover.
What you can expect from us
Single-storey extensions in Chelsea: questions
Which postcodes in Chelsea do you cover?
We cover SW3, SW5, SW7 and SW10, along with the rest of London. If your postcode is not listed, get in touch anyway: our coverage is UK-wide, and we can usually arrange a survey wherever you live. Enter your postcode in the guide-price tool or contact form and we will confirm.
How long does a single-storey extension take to build?
A single-storey extension usually takes 10–16 weeks on site, from digging foundations to final plastering. Bad weather, deep foundations or a complex roof can add time. Before work starts, allow several weeks for drawings, structural calculations and building control approval, and longer if planning permission is needed. We give you a stage-by-stage programme with your quote so you know what happens each week.
Is a flat roof or a pitched roof better on an extension?
A flat roof is quicker to build and usually costs less, gives a clean modern look and works well with a roof lantern. A pitched roof matches the main house, sheds water faster and allows a vaulted ceiling, but it costs more and its height can affect neighbours. Modern flat roofs in EPDM rubber or GRP last well when properly detailed. Planning rules on height can also decide the choice.
What does a typical extension build look like week by week?
A typical single-storey extension follows the same order: clearing the site and digging foundations, building walls up to roof level, then fitting the roof and windows to make it weathertight. Electrics, plumbing, insulation and plastering follow. The last stage is opening into the house, laying the screed and fitting doors and finishes. Building control inspects at key stages, and we book those visits for you.
How long will a single-storey extension take?
On a typical home, allow 10–16 weeks on site. Larger projects, older buildings and extra work found once things are opened up can extend that, while simpler jobs may be shorter. Your quote includes an estimate of time on site, and your point of contact keeps you updated on the programme as the work progresses.
Near Chelsea
Chelsea and its SW3, SW5, SW7 and SW10 postcodes are part of our coverage. Homeowners in Battersea, Fulham and Pimlico can also book a free survey with us and receive a written price for their job.