The Cost of a Single Storey Extension

All you need to know about building a single storey extension including costs of materials, labour and time frames.

The Cost Of a Single Storey Extension

What the job entails

If you find yourself stuck for space in your household and needing some extra room to alleviate the claustrophobia, then you essentially have two options: move house or extend the property. Making the right choice is obviously a big deal that could save you a lot of money, time and stress! Extending your home is often a cost-effective investment ' but not always. The location and demand in your area will often tell you if it is financially feasible to add an extension.

Building costs for an extension will also depend greatly on location as well as the size and the type of extension you will be planning. While moving house can be stressful and inconvenient, so too can extending your property. But for many, extending your current property is a cost-effective option that will save you from the mental torture that is the house buying and selling process!

The first step with any proposed extension is to work out your budget. Be realistic about how much you can afford and get an idea from local builders on typical per square foot costs. Don't forget to factor additional costs for fixtures and fittings, a decorating budget and professional fees. Be aware that there are often unforeseen expenses involved in building work when you come across something you weren't expecting before the work started, so always have a contingency fund of at least 10%. Then check whether or not you need planning permission with your local planning authority.

If planning is required, consult an architect and have professional plans drawn up. A major part of the project will be finding a reputable builder. Good builders are usually in demand so you need to book one in as soon as you can. Personal recommendation is the best way to find a builder if possible. But in any case always get a written quote before you make any decisions and make ensure this quote includes a time frame for the work to be completed and details of payment terms.

Even if you do not require planning permission, building regulation approval is essential, so your builder must make sure the local authority checks the work and ensures everything conforms to minimum standards so the building inspectors can sign it off. You'll need evidence of building regulation approval when you come to sell your home in the future. Before you start any building works, you must let your insurer know about any changes being made to the property as if your insurer isn't told about your plans, the home insurance policy may be invalidated.

An extension, even a simple single-storey extension, is still a major job and not really suitable fo a DIY project. However, there are ways to save money by doing at least some of the work yourself. For example, the excavation for the foundations is pretty straightforward and can save many hours of expensive professional labour. Then once the skin of the building is in place, much of the internal finishings like decorating, flooring, tiling, adding blinds and so on can be tackled by the average handyman which will greatly reduce the overall costs. But for the drawings and plans, the bricklaying, the roofing, the electrics and the plumbing and any plastering - hire professionals and save yourself major headaches and the possibility of costly building mistakes.

Problems with extensions invariably boil down to poor planning and/or cost-cutting. When extensions are professionally designed with architects drawings and then constructed using reputable builders - serious problems are not common. On the other hand, when homeowners try to save money by doing much of the planning and drawings themselves, along with hiring the cheapest builders possible - there is a good potential for the sort of disastrous problems that could have your story being featured on TV as a cautionary tale!

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The Cost of a Single Storey Extension

The average cost of materials for a single storey extension will vary due to many factors, mainly the size of the extension, but also the overall finish. Many homeowners these days are going for rear extensions with open plans and large glass windows. These glass windows are very expensive and can increase the price by 10's of thousands. A general rule of thumb for the cost of extensions is around £1000-£1500 per m2. Thus, the difference between a 25 sq. m extension to a 30 sq. m extension will be at least £5000.

Another cost factor is the use of the new extension. Commonly, homeowners use rear and side extensions as kitchens, which come with their own associated costs as most new kitchen suites will set you back at least £3000. Wheras an extended living room will likely need new furniture purchasing for it. Bathrooms are often incorporated into an extension also, with a new bathroom installation costing anywhere from £1000 to £10,000.

The average tradesman will usually charge around £150-£250 per day in labour and for an extension job, there will likely be at least two tradesman on site every day, whilst you can expect as many as 3-6 workers during the excavation and building stages. For a single storey extension you will need a few different trades to complete your job. An architect and a structural engineer are going to be needed to help with the planning applications, reports and to follow relevant building regulations. Builders and roofers will be needed of course, as well as an electrician and plumber or heating specialist for all of your energy and power requirements. Finally, you'll want it looking pretty so you'll need to hire a plasterer, decorator, tiler and floor fitter.

In terms of time frames, most extensions take at least 2 months, with weather and scheduling issues easily lenghtening the process passed the 6 months mark.

Below are some estimated costs of a single storey extension:

Extension Size Avg. Cost Duration
Small Single Storey Extension - 15 m2 £17,000 - £18,000 7-9 weeks
Medium Single Storey Extension - 25 m2 £30,000 - £32,000 8-10 weeks
Large Single Storey Extension - 45 m2 £40,000 - £45,000 10-12 weeks

Cost Breakdown

Individual costs of single storey extension at 25 m2 - Total Cost: £30,000






Waste Removal


Parliament has given the main responsibility for planning to the local planning authorities in your local council. So, if you have any questions, you need to contact your local planning authority. It is your responsibility for seeking planning permission and if it is required, you should have permission before any work starts.
Expect around 3-4 months from contacting an architect to getting written permission.
Certain types of work can be performed without needing to apply for planning permission. These projects can be classed as "permitted development rights". However, the proposed extension will need to comply with a number of specific guidelines before it can be classed as being permitted development rights.
Flat roofs got a bad name thanks to many of them being built quickly and cheaply 50 years ago and causing lots of problems since. But nowadays, flat roof extensions are extremely robust and they use less material so are much cheaper than a pitched roof. In addition, flat roofs can be used as a roof terrace adding outside living space (if suitably reinforced).
First of all, it's important to find out exactly why the application was rejected. It may well be that the overall proposals were fine, but there was a troublesome detail that caused the rejection. In this case, a new amended application should be enough to get the permission. Working with an architect or professional building company will greatly reduce the likelihood of being surprised by a planning rejection!
Single-storey extensions, particularly to the rear, are very popular and affordable. This sort of extension is a relatively straightforward building project and in most cases are classed as permitted development, meaning you don't need to obtain planning permission unless you live in a conservation area or listed building. However, to be on the safe side you should always check with your local planning authority before making any firm decisions about extending your home.