1. Keep your extension design simple

Curves and corners are costly to build, so keep your extension design as simple as possible. A rectangular or square footprint with a simple pitched roof will be cheapest. Design the build around off-the-shelf products, such as standard-size doors and windows.

Avoid any products that have to be made to order; choose materials that are readily available and easy to use. So, stick to cast concrete for the sub-floor; concrete blockwork for the walls; brick, render or timber cladding; and a softwood timber roof structure. Try rooflights instead of dormer windows and interlocking concrete tiles for covering the roof.

Click here to get a comprehensive extension cost calculation using our free extension cost calculator

Related articles: The ultimate guide to extending your home | 12 kitchen extensions all under £100k | Planning and costing your single storey extension

If possible, avoid complicated groundworks, such as building near to trees or drains and sewers, or other buried services, as these will increase groundwork costs. Remember, though, that if you live in a period or character property, your choices will be limited.

Glass box extensions lend themselves to simple design, whilst maximising natural light. CLICK HERE to see our guide on creating the perfect glass box extension

2. Save money with a DIY extension

Labour costs represent between half and two-thirds of the outlay for a typical extension, so if you can do some of the work yourself, there is great scope to make savings.

The easiest tasks to take on are labouring, decorating and landscaping, followed by some of the ‘second fix’ trades, such as kitchen and bathroom fitting, tiling and fixing architraves, coving and skirting boards.

Be careful only to take on work you have the time and skills to complete effectively, though. Read up in advance and never rush, as poor workmanship and wasted materials can make DIY a false economy. Unless you are an expert, hire professionals for the skilled work where the results will be on view all the time — for example, bricklaying and plastering.

infographic of ways to cut the cost of your extension

3. Project-manage the extension

A builder will add 15–25 per cent onto the total cost of labour and materials to cover their time for managing the project. You can save some of this cost by effectively taking on the role of building contractor yourself. This will mean liaising with your designer/architect and your local authority’s building control department, finding and hiring tradespeople, directing the work and supplying all of the necessary materials, plus scaffolding, skips and so on.

Though time-consuming, it can be very rewarding. To do it well, you need time and flexibility, plus confidence, management skills and some knowledge of construction. The job may take longer to complete overall, but the savings can be enormous.

4. Save on VAT

Most extension work will attract VAT at 20 per cent on labour and materials, but if you use self-employed tradespeople who each turnover less than the threshold for VAT registration, you will not be charged this tax — saving on labour costs. Second hand materials sold by private individuals on the internet will also be free of VAT.

Some types of work attract reduced-rate VAT at in any case — upgrading insulation and extending a building that’s been empty for two years, for example.

For more information, visit www.homebuilding.co.uk/reclaiming-vat.

5. Keep in with your neighbours

If you are building on or near the boundary of a neighbour, your extension will need to comply with the Party Wall Act (England & Wales) 1996. If a party wall settlement is required, it will cost around £700 per neighbour; more if they use their own independent surveyors.

You must notify your neighbours in writing about your extension plans eight weeks before you start. If you can get them to write back that they do not object, you can avoid using a surveyor to arrange a party wall settlement and save on fees.

It pays, therefore, to keep neighbours on board with your project, discussing plans and being considerate about any concerns they have.

6. Save on fees

While it is a false economy not to invest in design, some designers know how to keep down costs, while others only produce very complicated, if beautiful, plans that are very expensive to build. For a simple, low-cost build, find an architect, architectural technician, chartered surveyor or structural engineer who will produce planning and Building Regulations drawings for a fixed, all-in price. Look for a track record in designing low-cost projects.

If your extension can be designed to fall within your permitted development rights (see www.planningportal.gov.uk), no planning application will be required. This will save the planning fee of £172.

CLICK HERE to find out if your project needs planning permission

7. Avoid cowboys

There are plenty of cowboy builders (as in any trade) and they may vastly underestimate costs — through incompetence or, possibly, deliberately to secure a job. They may then ask for more money for changes or ‘extras’. They could even demand some or all of the money up front, leaving you with no comeback if they fail to complete the work to satisfactorily.

To avoid being ripped off, always ask for references — and check them. Never, ever pay for building work in advance; pay only for work that has been completed and that you are happy with.

For a small project, pay when the job is finished. For an extension, agree payments at set stages, or interim payments based on a verifiable list of labour and materials used up to date. Never hand over money for materials in advance. If a builder does not want to supply materials, buy them yourself.

8. Measure twice, but cut only once

Making changes or mistakes that waste labour and materials is a major factor in the final bill for many projects. Measuring everything onsite, rather than off your plans will help reduce wastage. Having accurate, scaled plans in the first place will help.

The more time spent at the design stage, visualising the end result, the more problems can be anticipated and headed off. Accurate and complete design will ensure that you have services, such as plumbing and wiring, in the right places, get floor levels right between rooms, and ensure that doors are hung to swing in the most space-efficient direction.

Once you have made up your mind, stick to it; changes always incur extra costs, often in ways that don’t reveal themselves until much later.

Over-ordering materials can also waste money, but it’s better to have slightly too much than to fall short and have problems making up the difference. This will incur delays and extra delivery charges; worse, you may not be able to find the exact same thing again.

9. Reuse, recycle and repair

Existing materials can be reused or sold rather than thrown in a skip. Old floorboards, doors, radiators, towel rails, kitchen units etc. can all be revived and reused, cleaned up and given a new finish. Sell or trade what you can’t use.

Buying salvaged materials – on the internet or from salvage yards – can be a lot cheaper than buying new. It will also introduce instant character. Second-hand items that offer good value include roof tiles, bricks, internal doors, timber floorboards, fireplaces and roll-top baths. Some people even hunt through skips.

Reducing wastage will also reduce costs for skip hire and disposal. Bear in mind, too, that as a private individual, you can dispose of waste in your local council tip for free.

10. Negotiate trade discounts

Find out where those in the trade buy their materials and aim to get the same wholesale/trade prices. Always negotiate and see if there is any discount for paying in cash — making sure you get a receipt. Bulk-buy from a single supplier for a further discount and ask about reduced delivery costs. Buying end-of-line deals will save you a fortune, especially on items such as carpets, units and appliances.

Getting the best deal will often mean moving away from the big brand names and finding equivalents without the price premium. If you are clever, however, you can save on price without compromising quality.

To keep down costs, stick with a basic specification — so go for radiator-based central heating, carpets for floors and standard white sanitaryware.