Despite the growing popularity of self-building, by far the most common way of achieving a suitable home is by extending. There are no official figures for the number of house extensions undertaken each year, as many do not require planning permission, but it is estimated that there are in the region of 100-200,000 projects, typically costing £30-70,000.

While many of these extensions are significant additions often to newly owned, run-down houses, the overwhelming majority are carried out by ordinary homeowners to ordinary homes — terraces, semis and modest detached houses. Consequently, moving out while work takes place is rarely an option and for most extenders, the aim is to get their improved spaces with the minimum fuss and in the quickest possible time.

Costs – What to Expect

Many people use cost/m² prices to give an estimate of what new build work will be. However, these are much less accurate on extension projects. The reason for this is that the expensive parts of a building job – the footings and roof structure – are evened out on a larger project by the amount of easier work and ‘dead’ spaces. There are also more unknowns. As a result, it doesn’t follow that the cost/² diminishes proportionately when you get down to projects of less than around 70² — so a 20² extension will not cost a tenth of a 200² new build.

In general, you should budget around £1,000-1,500/² for a two storey extension, meaning the typical project (adding extra living space, a larger kitchen and a bedroom) might cost between £40-100,000, with typical prices being in the £60-70,000 region (based on a 40² project floor area). For a single storey project, the cost/² might push up to between £1-2,000/², giving a typical cost of between £25-50,000, up to 25².

In general, the ‘construction’ element of a project will account for around 60% of the expenditure, while fittings will take up the remainder. However, this depends on the specification (there is huge variance in the price of kitchens, for instance).

Site Essentials

Homeowners don’t have any formal responsibilities under Health & Safety laws. Responsibility instead falls with the main contractor; domestic works are exempt under the Construction (Design & Management) Regulations, and the Health & Safety Executive is unlikely to be interested in a small project like an extension. However, there are practical issues of site management. Firstly, it’s important to establish the provision of toilet facilities. In truth, you won’t want builders traipsing through to use your facilities and you might agree with the main contractor for them to provide their own portable chemical toilet (they cost about £45/week to hire, including ‘service’). It’s also important to deal with how to dispose of (other, building-related) waste from site. Most people will opt for a skip — depending on the type of waste and the quantity, expect to pay £40-150 per skip load. Ensure, too, that the building team has access to water and electrics. While the builders are likely to have their own insurances covering their time on your site, it might be worth a call to your home insurance provider to make them aware.

How Fast Can It Be Done?

Design and build extension company Glassbox (one of its extensions is pictured above) incorporates an element of prefabrication into its extension projects, which are quintessentially contemporary in look, mixing render with plenty of glass. Owner Zig Choudry says that the level of prefabrication depends on each individual site. “In situations with terraced homes with limited access, it’s either opting for a crane to haul in our structural insulated panels (SIPs) at a cost of £2-5,000 and a lot of hassle, or being pragmatic about incorporating more traditional construction elements. You can’t ship big panels through someone’s house.”

That said, Zig says that he can achieve a complete extension in six to eight weeks. “It takes a week to do the groundworks and preparation, and we have got the main structural elements – steels, putting up the extension walls, etc. – down to another week. A further four to six weeks are taken up with the things we can’t reduce, such as screeding and plastering. It’s really down to being super organised and having a good experienced team on site — and putting plenty of men onto the job.”As a result, he admits, costs are a little higher than a traditional build, but the disruption is significantly reduced. Even so, all-in prices start from £25,000.

Another company offering quick build times and minimal site impact is Building for the Future Ltd, whose Rapyd Rooms extensions are made of prefabricated SIPs. “We use either mini piles or AB anchors to minimize disruption at foundation stage, and our installation team (we will also train a local builder) can get the shell of a two storey home up in a week. A completed project costs around £1,000/²,”says MD Gerry Woodham.

A Typical Extension Schedule

Here’s a typical schedule for a single storey* kitchen extension project (bear in mind that extension projects are difficult to generalise on, as the extent of remedial works to the existing structure varies so much):

 Week 1 Setting out, digging out footings and pouring concrete foundations
 Week 2 Building up to damp-proof course (DPC), drainage, installing DPC, concrete oversite/floor structure
 Week 3-4 Commence build up of exterior walls — in block and brick. Any internal walls built up
 Week 5 Installation of roof structure and begin covering
 Week 6 Installation of windows and doors
 Week 7 Commence first fix electrics and plumbing, carpentry runs
 Week 8 Break through to existing house and make good all joins, install steels etc.
 Week 9-10 Plaster out and dry
 Week 11-13 Second fix electrics, plumbing, fitting flooring, installing kitchen units
 Week 12 Snagging

*Expect a two storey extension to take three to five weeks longer due to the extra floor structure installation and additional wall construction.

The Great Time/Cost/Mess Trade Off
It’s important to understand that while extension projects can be cheap or non-disruptive or fast, they can rarely be all three. You can speed things up by using a prefab system or getting more men on site — but it will add to costs. Likewise, you can have a stress-free project but it will again add to the cost.

Our Sponsors