Ten steps to the perfect cost-saving extension, loft conversion or basement room

More people are deciding to improve their homes with extensions, loft conversions and basement rooms instead of moving. Michael Holmes of the National Homebuilding and Renovating Show gives his top tips for adding value and space to your home while keeping the costs down
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Easter marks the start of the home improvements season, with Londoners eager to create more space with extensions, loft conversions and basement rooms. Many have decided that staying put and improving their home makes more sense than moving on — saving on stamp duty, legal and mortgage fees and moving costs — especially because most new homes will need improvements anyway.

Michael Holmes, of the National Homebuilding and Renovating Show, is the man to turn to when you’re struck with spring DIY fever. Here he offers his 11 top tips for adding value and space to your home while keeping the costs to a minimum.

Floor-to-ceiling windows
© Alamy
Floor-to-ceiling windows are stunning in a loft conversion - but make sure the view is worth the cost

1. Loft conversions: keep it simple
Making use of your existing roofspace is usually the most cost-effective way to add space and value to your home.

Keeping things simple — inserting a rooflight with no alteration to the shape of the roof — will be much cheaper than building a dormer extension, let alone a hip-to-gable, or mansard roof conversion, but you’ll get less headroom and less usable space. Try to keep the stairs directly above the flight below to maximise space efficiency so that you don’t lose existing living space below. If you include bathroom facilities, try to put them where they can be easily connected to an existing soil pipe downstairs.

Many loft conversions require steel “purlins” to be inserted to support the rafters once the existing structural timbers have been altered. Using lightweight telescopic beams (telebeam.co.uk) will reduce costs considerably compared with having to hire a crane to bring in heavy steel beams.

A specialist design-and-build contractor, who can apply a standard design and engineering solution, will cost a lot less than a bespoke design from an architect.

2. Building an extension: something on the side
Extending into the side return to widen the kitchen at the back of a typical Victorian terrace house or semi is a classic design solution. Ask your neighbours if they want to extend because there are savings to be had by sharing a builder, especially on the shared party wall at the property’s boundary. A glazed roof over a single-storey side return is a nice addition to bring in light, but off-the-shelf rooflights will be a much more cost-effective solution.

Try to avoid building over an inspection chamber (manhole), as diverting the drain and adding a new chamber will cost £1,200-£1,800 extra. Avoid extending near any protected trees in a conservation area to save money.

3. A basement extension: digging deep
Extending under your home is likely to be your most expensive option and, because of the importance of having a guarantee on the waterproof tanking in a basement extension or cellar conversion, it’s not something that you can easily manage yourself with subcontractors.

Installing the basement extension under the garden instead of the house saves money, but you need planning permission if there is access from inside the house to the extension.

It is also much easier to build in the garden because you can use a machine such as a digger or mini-digger to excavate the garden, while workers under the house will be digging by hand. Even where access is restricted, a mini-digger can be craned into the back garden over the roof. Having easy access to remove the spoil from the dig will also help to reduce costs, rather than having to manhandle it through the house in wheelbarrows.

Basement extension
© Alamy
Basement extensions are the most expensive way to add space but can be made more affordable by excavating the garden rather than the cellar

4. The builders: price estimates and checking references
An estimate is a builder’s best guess at the cost of your project. It is not the final price. A written quote is a firm offer to undertake work for a fixed price.

However, there is no such thing as a fixed price in the building industry, unless you ensure that every nail and screw in the design and specification is written down and included in a contract. For a larger project, it is best to employ a quantity surveyor. Watch out for “exclusions” and make sure that you are comparing like with like when choosing your quote.

Ask all builders for a quote on the same format, with “prime cost” allowances for items such as bathrooms and kitchens, and “provisional sum” allowances for any “hidden costs” that cannot reasonably be known about until work starts.

When it comes to picking a builder, as well as checking anything available on paper, visit a prospective builder’s previous work in person. Never pay in advance for labour or materials and do remember that good builders are usually busy, so you will need to give plenty of notice for any work you want them to do.

Building the perfect extension
© Nigel Rigden
Keep your design square or rectangular so you can benefit from cheaper off-the-shelf materials
5. Square-shaped, not pear-shaped: no cutting corners
Avoid complicated room shapes, which will force up your labour and materials costs. Keep it simple: it will be easier and cheaper to build.

Square and rectangular rooms are typically the most cost efficient to construct, because you can use standard materials and off-the-shelf doors and windows.

6. Money talks: pay cash
Being prepared to pay with cash gives you the upper hand in negotiating the cost of a job with contractors and tradespeople. Some smaller firms and self-employed subcontractors may even offer a discount. Cash might secure discounts on items such as salvaged materials. But make sure you get a receipt.

Building the perfect extension
© Alamy
Let your artistry run riot with a custom design like this and costs will soar. Keep things simple to save money
7. Do it yourself: your time is money
Labour costs represent between half and two thirds of the budget for a typical home improvement project. The easiest tasks to take on yourself are labouring, decorating and landscaping, tiling and fixing architrave and skirting etc. Only take on work that you have the time and skills to complete effectively, or DIY will become a false economy.

8. Get a hard hat: become a contractor
A builder will add 15-25 per cent to the cost of labour and materials and sometimes a great deal more. By taking on the role of building contractor, you will have to talk with your designer/architect and the local authority building control department, find and hire tradespeople, direct the work and buy and supply materials, as well as arrange scaffolding, skips etc. If you are prepared do all that yourself, you will cut your costs considerably.

9. Party time: keep in with the neighbours
If you are building on or near the boundary of a neighbour, your extension will need to comply with the Party Wall Act (communities.gov.uk). If you do need a party wall settlement it will cost about £700 per neighbour and more if they insist on using their own independent surveyors. If you can get written agreement from your neighbours, you can avoid using a surveyor and save hundreds of pounds in fees. It pays, therefore, to keep your neighbours happy. Discuss your plans with them and be considerate.

10. Recyle and save: sell your old stuff
Before you throw out your old materials, think about selling them. Old floorboards, doors, radiators, towel rails, kitchen units etc can be sold or traded on the internet for second-hand roof tiles, bricks, timber floorboards, doors and fireplaces.

Renovating for Profit, by Michael Holmes, is published by Ebury and available on Amazon priced £17.28

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