The Build Extension Quotes Guide to Extending Your Home

1. The main types of extension

Most domestic projects fall into a handful of recognisable shapes, and the shape you choose drives both the cost and the planning route.

• Rear extension: added depth across the back of the house, usually to create an open-plan kitchen-diner. The most common project in the UK.
• Side return: fills the narrow alley beside a Victorian or Edwardian terrace. Modest floor area, large impact on light and layout.
• Wrap-around: a rear extension combined with a side return. Expensive per project but the best value per square metre gained.
• Double-storey extension: adds a bedroom or bathroom above new ground-floor space. Roughly 60 to 70 per cent more than a single storey of the same footprint, not double.
• Garage conversion: the cheapest usable space in most houses, provided the existing slab, walls and damp proofing are sound.
• Porch or entrance hall: small, quick, and often permitted development if under three square metres.
• Orangery or conservatory: a glazed room with a solid perimeter and a lantern roof. Cheaper than masonry, but check heating and shading.
• Loft conversion: not strictly an extension, but the usual alternative when the garden cannot be lost.

2. Planning permission vs permitted development

Permitted development rights let many homeowners extend without a full planning application. The limits are specific, and they are the first thing to check.

Flats and maisonettes have no permitted development rights at all. Listed buildings, conservation areas, national parks and areas of outstanding natural beauty are all restricted, and rights may also have been removed by an Article 4 direction.

• Single-storey rear extensions: commonly up to 3m deep on an attached house and 4m on a detached one, with larger depths possible under the prior-approval neighbour consultation scheme.
• Height: normally a maximum of 4m to the ridge, and no higher than the existing roof.
• Within 2m of a boundary the eaves must not exceed 3m.
• Side extensions must be single storey, no more than half the width of the original house.
• Double-storey rear extensions must not extend more than 3m and must sit at least 7m from the rear boundary.
• No more than half the land around the original house may be built on.
• Building regulations approval is required whichever route you take, and it is separate from planning.

3. The consents and paperwork you will need

Even a modest extension generates a folder of documents. Getting these in place early is what keeps a build on programme.

• Planning permission or a lawful development certificate confirming permitted development.
• Building regulations approval, via full plans or a building notice, signed off with a completion certificate.
• Structural engineer's calculations for beams, lintels and foundations.
• A party wall award where you build on or near a shared boundary, usually served two months before work starts.
• Build-over agreement from the water authority if you build within 3m of a public sewer.
• SAP or thermal calculations where there is a large amount of glazing.

4. What actually drives the cost

Floor area sets the ballpark; everything below moves the figure within it. Two identical footprints can differ by 40 per cent once site conditions and specification are known.

• Access: a terraced site where every barrow of muck is carried through the hallway costs noticeably more than a detached plot with side access.
• Ground conditions: clay, trees, sloping plots, old drains or a high water table all deepen foundations.
• Structure: the more wall you remove, the more steel and temporary propping is needed.
• Glazing: bifold and sliding doors, roof lanterns and structural glass are the most common budget overrun.
• Fit-out: a new kitchen, bathroom, underfloor heating, joinery and flooring are frequently excluded from headline builder prices.
• Services: moving a boiler, upgrading a consumer unit or relocating drainage adds cost quietly.
• Region: London and the South East typically run 15 to 30 per cent above the national average.
• VAT: most extension work on an existing home is standard rated, so check whether a price includes it.

5. How our estimate is calculated

Our figure starts from a rate per square metre for a standard builder's finish, then adjusts for the type of extension, how the property is attached, the property type and the number of storeys. Selected extras such as a kitchen or bifolds are added as fixed amounts, and the result is shown as a range rather than a single number.

It deliberately assumes a standard specification. If you plan an architect-led, high-specification build, expect to be above the top of the range.

6. The build stages, in order

A typical single-storey extension takes 10 to 16 weeks on site, with design and consents adding 8 to 16 weeks before that. A double storey usually runs 16 to 24 weeks.

• Design, measured survey and drawings.
• Planning application or permitted development check.
• Structural calculations and building control submission.
• Tendering: three comparable quotes on the same drawings.
• Site setup, demolition and groundworks.
• Foundations, drainage and slab, with building control inspections.
• Superstructure: blockwork, steels, roof structure and covering.
• Windows, doors and making the shell watertight.
• First fix electrics and plumbing, insulation and plasterboard.
• Plastering, screed and second fix.
• Kitchen or bathroom fit-out, decoration, snagging and sign-off.

7. Getting comparable quotes

Comparing quotes is only meaningful when every builder has priced the same job. A cheap quote almost always excludes something.

• Issue identical drawings and a written specification to every builder.
• Ask for a breakdown by stage rather than a single lump sum.
• Confirm what is excluded: kitchen, flooring, decoration, skips, scaffolding, VAT.
• Check public liability insurance and any warranty or guarantee cover.
• Agree a stage payment schedule tied to completed work, never a large payment up front.
• Use a written contract, such as a JCT Home Owner contract, and hold a retention until snagging is done.
• Take up two recent references and visit a finished job if you can.

8. Living through the build

Plan for disruption rather than hoping it will be brief. Most households manage with a temporary kitchen for six to ten weeks on a rear extension.

• Set up a temporary kitchen with a microwave, kettle and fridge before the existing one is stripped out.
• Agree working hours, parking, skip location and site tidiness in writing.
• Tell neighbours in advance; it prevents most disputes.
• Notify your buildings insurer, as many policies require it for structural work.
• Keep a shared snagging list and photograph everything before it is covered up.

9. Common mistakes to avoid

The same handful of errors account for most extension overruns we hear about.

• No contingency: hold back 10 to 15 per cent for the unknown.
• Choosing on price alone, then paying twice to put things right.
• Deciding the kitchen layout after the drawings are approved.
• Forgetting natural light: a deep rear extension needs roof glazing.
• Ignoring heating and insulation, so the new room is unusable in winter.
• Skipping the party wall procedure and losing weeks to a dispute.

10. Will it add value?

A well-built extension in the right area usually returns a meaningful part of its cost, and additional bedrooms or a kitchen-diner tend to perform best. Be aware of the local ceiling price: beyond a certain point a street will not support further value, however good the build. Losing all of a small garden, or squeezing out a parking space, can reduce appeal.