How Much Does a House Extension Cost in Kent?

Typical cost per square metre in Kent

For a straightforward single storey extension in Kent, most homeowners should budget somewhere between £2,200 and £3,200 per square metre for the build itself, finished to a plastered and decorated shell with basic electrics and plumbing. A more involved design with large glazed openings, a bespoke roof or a higher specification of finish will push that towards £3,500 per square metre or more.

Kent sits a little above the national average because labour and material costs track closely with London and the M25 corridor. Maidstone, Sevenoaks and Tunbridge Wells tend to price higher than Thanet or Swale, mainly down to local trade rates and access rather than any difference in the building work itself.

• Single storey rear extension, 20 sq m: roughly £45,000 to £65,000
• Single storey side return on a Victorian terrace, 12 to 15 sq m: roughly £35,000 to £50,000
• Double storey extension, 40 sq m across two floors: roughly £80,000 to £120,000
• Wraparound extension with open plan kitchen, 35 sq m: roughly £85,000 to £115,000

What is included and what is not

A builder's quotation usually covers foundations, drainage connections, the structural shell, roof, windows and doors, first and second fix electrics and plumbing, plastering and basic decoration. What it often excludes is the kitchen itself, bathroom sanitaryware, flooring, underfloor heating, bifold doors above a standard size and anything relating to the existing house such as rerouting a boiler flue or moving the consumer unit.

On top of the build you will need to allow for professional fees. Architectural drawings for a typical Kent extension run from £1,500 to £4,000 depending on whether you use a technician or a chartered architect. Structural engineer calculations are usually £500 to £1,200. Maidstone Borough Council charges around £258 for a householder planning application, and Building Control fees through the council or an approved inspector typically fall between £600 and £1,200 for an extension of this scale.

The things that quietly add thousands

Ground conditions are the biggest surprise on Kent sites. Much of the county sits on clay, particularly around Maidstone, and where there are mature trees nearby Building Control may require deeper foundations or piling. That alone can add £5,000 to £15,000 compared with standard strip footings. Homes on the chalk of the North Downs are generally easier, but old chalk workings and dene holes do turn up.

Drainage is the other common one. If a public sewer runs within three metres of your new foundations you will need a Build Over Agreement from Southern Water, which involves a fee, a CCTV survey and sometimes a redesign. Steel work for open plan layouts, removing a load bearing wall between the extension and the existing kitchen, and matching period brickwork on older Kent ragstone or Wealden properties all move the price upwards.

• Piled or deepened foundations due to clay and trees: £5,000 to £15,000 extra
• Build Over Agreement and drainage diversion: £1,500 to £6,000
• Structural steels and knock through into existing rooms: £2,000 to £8,000
• Bifold or sliding doors over 4 metres wide: £4,000 to £10,000
• Underfloor heating in screed: £1,200 to £2,500 for a typical extension

VAT, contingency and timescales

Extensions on existing homes attract VAT at 20 percent, so check whether the figure you have been quoted includes it. A £50,000 quote that turns out to be plus VAT is a £60,000 job, and that catches people out every year. Always ask for the figure in writing and confirm whether it is fixed price or an estimate that will be remeasured.

We would suggest holding back a contingency of around 10 percent of the build cost for the unexpected, whether that is rotten timbers found on opening up, an unexpected drain run or a change of mind on the layout once the walls are up. A single storey extension in Kent typically takes eight to fourteen weeks on site, with double storey work closer to four to six months, and planning approval adds another eight to ten weeks before anything can start.

How to get an accurate price rather than a guess

The most reliable way to compare quotes is to have drawings and a structural design prepared first, then send the same pack to two or three builders. Quotes based on a sketch and a conversation are not comparable, and the lowest one is usually the one that has left something out. Ask each builder to itemise what is included, what is provisional, and what they have assumed about the ground and drains.

Check that whoever you appoint carries public liability insurance, will handle Building Control inspections at each stage, and can give you addresses of similar extensions they have completed locally. A builder who works regularly with Maidstone Borough Council and the surrounding authorities will know what the inspectors expect and can save you both delay and cost.