Building an extension to your house can be a simpler alternative to moving when you’re in need of more living space, or if you simply want to modernise your home to make a kitchen diner for example. But how much does a house extension cost? What are the planning steps you need to take? and what are you permitted to build without planning permission? We’ll take a look at these questions and more in our guide to extending your home below.
First of all we’ll look at the costs involved. Typically, a house extension costs between £1,400 and £3,000 per m² (excluding VAT) which includes architects and planning fees, ground works, building, plastering, plumbing and electrical work. Prices vary based on the specifics of the build and location within the country. As an example an extension in London will cost anything from 20% more than a comparable build in the North East or Wales due to higher operating and material costs.
Single Storey Extension Cost
Generally, single storey extensions cost more per square metre than a double storey extension because certain base costs (like foundations and roofing) do not double when you add a second floor. In theory a triple storey would cost less still. In practise the difference in cost may be small though.
The price per square metre also depends on the size of the property. There are basic costs such as ground works that don’t increase too much with size and hence the cost per square metre will decrease as the floor area increases.
Total cost of project and m2 costs for having a single floor extension added to your home. The area of the house is usually specified by its internal floor area though sometimes it can be the external area. A simple construction may be just a shell structure (basic plumbing, electrics and building work) whereas many people will specify the actual cost i.e. after you have put in all the fittings (e.g. new kitchen or bathroom/shower suite) furnished and decorated (including carpets or other type of flooring) the extension.
| Extension Size | Typical Cost | Extension Cost per Sq/M |
|---|---|---|
| Small 3m x 4m (12m2 ) | £28,900 | £2,400 per sq/m |
| Medium 4m x 5m (20m2) | £39,900 | £1,995 per sq/m |
| Large 5m x 6m (30m2) | £54,000 | £1,800 per sq/m |
Double Storey Extension Cost
The total area of the job in square metres is the sum of the area of both the lower and upper floor. Usually it is considered to be the internal floor area rather than the external one. Obviously the price can vary considerably depending on the level of finish varying from a basic build which may just be for a shell to a complete finish including kitchen and bathroom fittings as an example. In general it is found that double storey extensions work out to be cheaper than single storey ones in terms of m2 costs – again depending on the level of finish.
| Extension Size | Typical Cost | Extension Cost per Sq/M |
|---|---|---|
| Small 3m x 4m (24m2 ) | £46,650 | £1,944 per sq/m |
| Medium 4m x 5m (40m2) | £63,835 | £1,595 per sq/m |
| Large 5m x 6m (60m2) | £85,870 | £1,430 per sq/m |
Factors Influencing House Extension Costs
The price for a home extension depends on several factors.
- Location within the country
- Availability of tradespeople
- Building requirements (e.g., specific brick or stone)
- Special foundation needs
- Extent of self-conducted work
- Inclusion of electrical/plumbing work
- Presence of a fitted kitchen
- New bathroom, shower room, or WC
- Extra finishes (e.g., tiled floors, walls, render)
- Flat or pitched roof
As an example the final cost of the project could comprise.
- Plannings costs – £1,500 – £5,000
- Shell build costs – £1000 – £1,800 per m2 (includes windows, external doors, and roof)
- Fixtures and fittings – £600 – £1,200 per m2 (includes electrical, plumbing, plastering, and flooring)
- Decoration finishing – £40-60 per m2
The fixtures and fittings aren’t just electrical and plumbing work but all the plastering and floor work. The shell build includes the windows and external doors plus roof.
How Can You Start Planning Your House Extension Today?
- Sketch preliminary plans or ideas.
- Consult with an architect for detailed drawings.
- Apply for planning permission (if required, see below).
- Obtain quotes from builders.
- Sign a contract with a builder. (Tip: Consider breaking down payments into stages based on job completion.)
- Ensure building regulations approval.
To help estimate your costs, try our house extension calculator. It takes into account external dimensions, roof type, and number of storeys.
What are the Different Types of House Extensions & Conversions?
- Loft conversion: If an external house extension is not practical for whatever reason you could consider a loft conversion. Provided you have the loft space to accommodate the requirements for access it can be a great way to add extra room at a lower cost, use our loft conversion calculator to understand how much a conversion will cost. Don’t forget improvements like this that make use of unused space are a great way to increase your home’s value.
- Garage conversion: Similar to the above a garage conversion can increase your home’s value and be a relatively straightforward and lower cost option compared to both a loft conversion and full house extension. Often used as a playroom, gym, home office or ground floor bedroom there are many options to convert an often unused and unloved space.
- Kitchen extension: A very popular option is to merge an adjoining kitchen and dining room in conjunction with a rear extension, add glass sliding or bi-fold doors to create a large, light and airy family kitchen. Use our extension cost calculator to get an idea of costs involved.
- New or replacement Conservatory: Another lower cost option to extend your home is to add a conservatory, costing from £10,000 with many modern designs and styles to choose from it can be a faster and simpler way to add living space without going for a full brick built house extension.
Planning Permission & Permitted Development for Rear House Extensions
Are you wondering, “What size rear extension can I build without planning permission?” or “Do I need council approval before building a rear extension?” You’re not alone! It can become overwhelming when searching for answers but it’s actually quite simple for extensions up to 3m or 4m.
Key Rules for Rear Extensions
- Terraced and Semi-Detached Houses: A single-storey rear extension can extend up to 3m without planning permission.
- Detached Houses: The limit for single-storey rear extensions is 4m.
- Double-Storey Extensions: You are restricted to a 3m extension for all property types.
These rules apply for rear extensions to houses and bungalows, but not to flats.
Check for Conditions Before Building
Before starting your build, ensure there are no restrictions on your property that could impact your plans. These restrictions are particularly common on newer properties or may have been placed during a previous planning approval. If such conditions exist, you may need to apply for full planning permission.
Do You Need Permission?
As long as your proposed rear extension complies with permitted development rules, you won’t need to seek formal planning permission before building. Once your extension is complete, you can optionally apply for a Lawful Development Certificate (LDC), which can be beneficial when selling your home.
Don’t Forget Building Regulations and Party Wall Agreements
Keep in mind that permitted development rights don’t exempt you from other requirements such as building regulations or Party Wall agreements. Make sure to have these covered to avoid any issues during or after construction.
User submitted single storey extension costs
These are the prices we have received for the total cost either per metre squared or for the whole job of building a double storey extension onto your property. Also, pay attention in the detailed comments to what was and wasn’t included in the build costs.
| Extension type | Roof type | Approx size (m²) | Description | Price | Units | Region | Updated |
|---|---|---|---|---|---|---|---|
| Single storey | Pitched roof | 21m² | Pitched roof extension on existing flat roof above garage and kitchen to 3 bed 60’s detached house. Added 1 extra bedroom and shower room plus extended an existing bedroom. Rendered finish with concrete tiled roof. | £42,800 | Total price | East Midlands | 2024 |
| Single storey | Flat roof | 23m² | ground floor 2.650 metres by 8.700 metres goal post shape. 2 room. play room and kitchen knock into existing kitchen to in large. first floor. bedroom. en-suite knock into existing bedroom 4 radiators | £35,000 | Total price | East Midlands | 2023 |
| Single storey | Pitched roof | 15m² | 3m x 5m external size price for basic shell of a pitched roof single storey extension | £2,300 | Per m² | East of England | 2023 |
| Single storey | Pitched roof | 24m² | Single storey extension. Remove existing conservatory and build a 24 sq m extension with pitched roof to house modern kitchen/diner. Included all plastering, decorating, electrics, tiling and kitchen fitting with island. | £41,600 | Total price | London | 2023 |
| Single storey | Flat roof | 56m² | Extension 7000×8000 2 radiators bifold door 5100×2300 pitched roof with 4 velux 780×980 vailant boiler steel supported roof | £96,000 | Total price | South East | 2023 |
| Single storey | Flat roof | 19m² | Single storey (supply and build) included kitchen fit out (labour only) | £2,100 | Per m² | South East | 2023 |
| Single storey | Pitched roof | 15m² | flemish bond single storey extension, basic plumbing and electrics, gas central heating | £1,650 | Per m² | East of England | 2023 |
| Single storey | Flat roof | 20m² | 4m x 5m kitchen / dining area. 4 m of bi-fold doors + under floor heating, full decoration, electrics & plumbing. | £41,000 | Total price | South East | 2023 |
| Single storey | Flat roof | 18m² | Had a rear kitchen extension, 6M x 3M all electrics, & plumbing, plastering, etc, the full Monty, we supplied rads & fittings, included a knock through and some RSJs, extra £350 to have it painted but worth it. | £1,700 | Per m² | North West | 2023 |
| Single storey | Pitched roof | 21m² | Single storey rear l shape extension, lounge extended by 3m, upvc french doors 1900mm with sidelites & top openers, kitchen area by 6m, two windows, part of existing kitchen turned into a shower room, with window, new 4 ft high retaining garden wall, with steps 30 ft long, 25m2 600mm by 600mm slabs for patio area. existing hallway enlarged into old storm porch area. New 2.4m by 1.0m front brick porch, with door and sidelites. two side entry timber gates, concrete marley roof tiles, kitchen and shower room fitting & tiling included, new valiant 32kw boiler. | £41,200 | Total price | West Midlands | 2023 |
| Single storey | Pitched roof | 2m² | Downstairs toilet 1m x 2m with macerator, lean to roof, toilet, basin, plumbing, electrics [light/switch], floor tiles, door x 1, small window, paint walls and ceiling, skirting. | £9,340 | Total price | East Midlands | 2023 |
| Single storey | Pitched roof | 22m² | Single storey side extension. Two rooms with radiators and patio doors and one unity room with washing machine fittings and extractor fan. Plastered throughout, tiled pitched room. Very good job. 22m2 | £23,500 | Total price | North West | 2023 |
| Single storey | Flat roof | 36m² | 6m x 6m kitchen dining extension in block work with flat roof, french doors and one window. Fit one radiator, plaster ready for decorating. Fit kitchen supplied. | £67,000 | Total price | London | 2023 |
| Single storey | Pitched roof | 14m² | Single storey to rear of house, extension to kitchen knock down outside wall fit rsj. Extension bedroom, en suite very small. Side of building small utility room. Block and brick, no elec or central heating supplied by builder. | £62,000 | Total price | South East | 2023 |
| Single storey | Flat roof | 15m² | An average rear/side extension measures around 3X5 sq metres. All materials and labour. The walls are all insulated, radiators supplied and fitted, plumbing and electrical is done by certifieds. Boiler, kitchen units, appliances not included. | £37,600 | Total price | London | 2023 |
User submitted double storey extension costs
| Extension type | Roof type | Approx size (m²) | Description | Price | Units | Region | Updated |
|---|---|---|---|---|---|---|---|
| Double storey | Pitched roof | 40m² | double storey extension approx 40 square metres. bedroom upstairs with en-suite, and 3 rooms downstairs including wc. | £1,400 | Per m² | East of England | 2023 |
| Double storey | Pitched roof | 11m² | Two storey domestic side /rear extension. Utility/laundry wc at ground floor and family bathroom at first floor. Internal size 2.2x5m with traditional cavity wall and pitched roof over. Including tiling to both rooms ,lighting and bathroom fittings | £39,000 | Total price | West Midlands | 2023 |
| Double storey | Pitched roof | 40m² | Double storey side extension (8m x 5m) plus loft conversion. 3 bathrooms which 2 of were wet rooms. Full new rewire/central heating system, underfloor heating, new kitchen supplied and fitted, house also rendered to complete finish. | £122,000 | Total price | West Midlands | 2023 |
| Double storey | Flat roof | 32m² | demolish old extension build new two storey extension raising first floor level and raising second floor roof & ceilings to suit existing building floor area 32m2 | £1,300 | Per m² | North East | 2023 |
| Double storey | Pitched roof | 54m² | 6.6m x 4.1m, downstairs toilet/shower room , small utility, large dining room, upstairs bedrooms, knock through upstairs to continue hallway, plaster-boarded, plaster finish, all electrics, plumbing included, chipboard flooring to first floor, pitched roof with gable. | £71,000 | Total price | Cymru Wales | 2023 |
| Double storey | Pitched roof | 48m² | 88,500 including VAT to build 10 by 4.8 metres double story extension. This does not include any bathroom or kitchen fittings. This absolute basic. Expected to add another 35k for ground floor kitchen and upstairs bathroom at least | £88,500 | Total price | North West | 2023 |
| Double storey | Flat roof | 52m² | Including Foundations, Walls, windows, roof, plastering, 26m2 per level | £65,000 | Total price | Cymru Wales | 2023 |
| Double storey | Pitched roof | 48m² | 48m2 double storey extension complete | £105,000 | Total price | South East | 2023 |
| Double storey | Pitched roof | 61m² | New porch plus two storey extension to a semi-detached ex council house, comprising one lounge/diner, two bedrooms and an en-suite. Two standard doorway size external wall knock-throughs were required. 1.2m foundations. Pitched roof + gable end. External dimensions of extension: 4.5m x 6.8m. External dimensions of porch: 2.4m x 1.5m. Price included demolition of existing single storey extension, five external windows, one standard external door, a large set of patio doors. All electrical, plumbing, carpentry, plastering and tiling work included. Price also includes all planning and building regulation costs, which we sorted out ourselves (approximately £2500). Painting, carpets, furnishings etc. are not included. Our builder saved money by re-using materials where possible. There was also no VAT to pay, because his turnover falls below the VAT threshold (we paid for all materials and other trades directly). | £57,700 | Total price | South West | 2023 |
| Double storey | Pitched roof | 30m² | Complete job side extension double storey plus porch and rear extension 5 x 3m full plaster electrics rads in each off rooms standard basic stuff. | £51,000 | Total price | North East | 2023 |
| Double storey | Pitched roof | 44m² | Two storey extension, 22m2 per floor Ground floor – lounge two windows north & west elevation, double patio doors to south leading into garden. Fire place on east side of room for wood / duel fuel burner (connecting to existing fire place & chimney from room next door). First Floor – master bedroom with en suite shower, creating corridor from existing building. Relocate existing en suite to master bedroom. Quotes include first fit and second fit & making good. | £62,000 | Total price | South West | 2023 |
| Double storey | Pitched roof | 91m² | 5mtr sq double storey extension, single storey utility room, cloakroom and kitchen plus front porch. In all 91sq metre project build. Include all internal modifications, electrics and plumbing. All walls painted to top coat. | £133,000 | Total price | West Midlands | 2023 |
| Double storey | Pitched roof | 43m² | 43 square metre extension bathroom utility wet room porch kitchen new heating plumbing electrics warm flat roof gravel dashed landscaping fencing painting outside of house finished project | £57,700 | Total price | South West | 2023 |
