Most homeowners in Kent start with a number in their head and get a shock when the first quote lands. The honest answer is that a single storey extension is not one price but a stack of costs, and a few decisions early on move that stack by tens of thousands. Here is how those numbers break down locally, and where the money actually goes.
For a straightforward rear extension in the Maidstone area, expect somewhere between £2,200 and £3,000 per square metre for the build itself, finished to a decent but not luxury standard. A typical 20 square metre rear extension therefore sits around £44,000 to £60,000. Kent sits above the national average but below inner London rates, and prices ease slightly as you move towards Ashford and the Medway towns compared with the Sevenoaks and Tunbridge Wells end of the county.
Those figures assume a shell with plastering, electrics, heating, flooring and basic decoration. They do not include a kitchen, bathroom, bifold doors beyond a standard set, or landscaping to make good the garden afterwards. Budget separately for those and you will avoid the most common mid-build squeeze.
Groundworks are the single biggest unknown. Much of Kent sits on clay, and clay near mature trees means deeper foundations. A standard trench fill at around 1 metre can become 2.5 metres or more, and each extra metre of depth across the footprint adds concrete, muck away and labour. On some Maidstone plots we also hit ragstone or old chalk workings, which changes the approach again.
Drainage is the second. If a public sewer runs under or near your extension you will need a build over agreement from Southern Water, which takes time and sometimes means diverting the run. Existing manholes inside the footprint almost always need relocating.
Then there is the roof and glazing. A flat roof with two rooflights is far cheaper than a vaulted ceiling with a 4 metre structural opening and aluminium sliding doors. Steel beams, the structural engineer's calculations and the temporary propping to install them are a genuine cost, not an upsell.
The build price from your builder is rarely the total project cost. Professional fees and statutory charges typically add 8 to 12 per cent on top, and they arrive before any bricks are laid.
If your neighbours share a wall or you are digging within 3 metres of their foundations, the Party Wall Act applies. Most neighbours sign a simple consent, but if one dissents you are paying for surveyors on both sides.
Many single storey rear extensions fall under permitted development, which saves both fee and waiting time. The limits are 4 metres deep on a detached house and 3 metres on a semi or terrace, with larger projections up to 8 and 6 metres possible under the prior approval route. Height is capped at 4 metres, and 3 metres if within 2 metres of a boundary.
That all changes in a conservation area, on a listed building, or where permitted development rights have been removed, which is common on newer estates around Kings Hill and parts of Allington. Within the Kent Downs National Landscape the design scrutiny is noticeably tighter. Check your title and the council's planning map before you fall in love with a design, because a refused scheme redrawn costs months.
Three quotes that differ by £20,000 usually differ in what they include, not in greed. Ask every builder to price the same written specification: foundation depth assumed, roof construction, glazing make and model, number of sockets, floor finish, and who makes good the garden.
Be wary of any quote that is a single figure on one page, and of large deposits. A staged payment schedule tied to completed work protects you, and a builder confident in their pricing will not object to it.
A typical rear extension takes 10 to 16 weeks on site once work starts, with groundworks and the roof being the weather sensitive stages. Add two to three months beforehand for drawings, approvals and scheduling.
In most of Kent an extension adds value, but the uplift rarely exceeds the spend on higher value homes where the ceiling price for the street is already close. Adding usable kitchen and dining space to a three bed semi tends to return better than extending an already large house.
Yes, in most cases, though you should expect to lose the use of your kitchen or back rooms for several weeks and to live with dust and noise. Many homeowners set up a temporary kitchen in another room and plan a short break around the messiest phase.
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