Ask three builders what a rear extension costs and you'll get three answers, because the number depends far more on your ground, your drainage and your finish than on the square metres. Here's how the money actually breaks down on Croydon jobs, from Purley to Thornton Heath, and where budgets usually go wrong.
For a straightforward rear extension in the Croydon area, expect roughly £2,200 to £2,900 per square metre for the build itself, taken to a decent standard with plastered walls, sockets, heating and a flat roof. A typical 4m x 5m rear extension therefore lands somewhere around £44,000 to £58,000 before VAT, kitchen units or flooring.
That range is wide for a reason. The lower end assumes good ground, easy access and a simple rectangular footprint. The upper end covers structural steels, a lot of glazing, or a job where everything has to come through the house because there's no side gate.
Bear in mind these figures exclude the kitchen. Fitting out a new kitchen space realistically adds £8,000 to £25,000 depending on whether you're buying trade units or something bespoke, and that's the single biggest variable in most people's final spend.
Foundations are the usual culprit. Large parts of the borough sit on London Clay, and where there are mature trees nearby, and Croydon has plenty of tree-lined streets in Shirley, Sanderstead and Addiscombe, building control will often want deeper foundations. Going from a standard 1m trench fill to 2.5m or 3m can add £4,000 to £9,000 on its own, and you rarely know for certain until the trial hole is dug.
Drainage is the second one. If your extension crosses or sits within 3m of a public sewer, you'll need a build over agreement from Thames Water, which costs a few hundred pounds in fees and can take four to six weeks. Moving an existing manhole or building a new inspection chamber adds £1,500 to £3,500.
Access matters more than people expect. A mid terrace in Selhurst or South Norwood with no rear access means every barrow of muck and every bag of ballast goes through the hallway. That's slower, needs more protection, and typically adds five to ten per cent to labour. Skip permits from the council for on-street placement are another running cost, usually charged weekly.
Many single storey rear extensions in Croydon fall under permitted development, up to 3m deep on a terrace or semi and 4m on a detached house, or up to 6m and 8m respectively under the larger home extension process with a neighbour consultation. If you're within the Conservation Areas around Park Hill, Waddon Court Road or Upper Norwood, or your house has an Article 4 direction, assume you'll need full planning permission.
The soft costs add up faster than people plan for. A householder planning application is currently around £258, building control through Croydon or an approved inspector runs £900 to £1,600, structural engineer's calculations for steels are £600 to £1,200, and a Party Wall surveyor for a semi or terrace is commonly £1,200 to £2,000 per neighbour if they dissent.
The cheapest quote is usually the least detailed one. Ask every builder to price the same specification and to state clearly what's provisional: foundation depth, kitchen supply, floor covering, glazing units and decoration are the five items most often quietly left out.
Look for a provisional sum against foundations rather than a flat figure, because an honest builder cannot know the depth in advance. A quote that promises a fixed foundation cost before ground has been opened is either padded or heading for a variation later.
Payment should be staged against completed work, never a large sum up front. A deposit for materials is reasonable; a third of the contract value before anyone arrives is not.
Most single storey rear extensions in Croydon take 10 to 16 weeks on site, assuming reasonable weather and no foundation surprises. Add four to eight weeks beforehand for planning, engineer's drawings and Party Wall notices.
Yes, extensions to existing homes are standard rated at 20 per cent, so check whether a quote includes it. Some conversions of a property that has been empty for over two years qualify for a reduced rate, which is worth asking about.
In many parts of the borough a well built extension that adds usable kitchen or family space broadly returns its cost, particularly where it takes a two bedroom house to three usable reception spaces. It's rarely a profit exercise on its own, so build it because you want the space rather than as an investment.
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