1. Correctly allocate your budget

Don’t forget that, once your extension is actually built, you will need some money to complete the finishing work. Having a badly finished interior, even on an extension under £50,000, will damage the resale value.

You want to invest in details that will make the space look more expensive than it was. A feature pendant light or good quality door hardware can lift the appearance of a room, and make sure all of the decorative finishes, plastering, paint and flooring, are all done to the highest standard you can afford.

2. Pay attention to project management

‘Knowing exactly what the project will cost before you start is essential when funds are limited,’ says Rachel Haynes, director of Thread Architects. ‘If you are leading the project yourself and employing sub-contractors, you may need to provide all the materials to the builders as and when they require them. You will need to make sure you understand building terminology, what the builder will be doing and when, and what they’ll need to complete each task. All this takes forward planning as manufacturing and delivery times need to be factored into the process.

If you don’t have the time or skills to project manage, appoint a competent builder who will take on the day-to-day running of the site. Have a contract in place for the work that sets out the process and costs. An architect or separate project manager could help you administer the contract and/or oversee the work. They can answer technical queries and advise on issues relating to changes or extras. This may involve more consultancy fees but a well-managed project will help ensure the build doesn’t increase.’

David Salisbury conservatory on house in Somerset

This bespoke conservatory from David Salisbury adds a light-filled and very versatile space to this home in Somerset. It cost £40,000 to build

3. If you have the skills, do it yourself

‘There is nothing a proficient DIYer cannot do themselves when extending or improving their own home,’ says experienced home renovator Michael Holmes. ‘But while it is possible to reduce labour costs through DIY, as the client you must keep up to speed with any decision making necessary. Holding up other trades can expensive if a lack of decisiveness means delays or abortive work.’

Easy DIY jobs

For those with time but limited skills:

  • Site tidying and cleaning
  • Labouring (loading out roof tiles or blocks, mixing mortar etc.)
  • Landscaping and planting

For those with basic carpentry skills:

  • Second fix carpentry, for example fitting skirting and architrave
  • Building fitted furniture and kitchen units
  • Fixing curtain poles and hanging curtains

For those with basic electrical skills:

  • Fitting sockets and switches (second fix)
  • Adding light fittings (second fix)

For those with basic DIY skills:

  • Preparation and decoration (painting and papering)
  • Ceramic wall tiling

4.  Avoid making design changes down the line

‘To limit professional fees, try to “freeze” an agreed design at an early stage, when the drawings have the level of detail necessary to be submitted for planning permission or a certificate of lawful development,’ says architect Alan Cronshaw of ACRONYM Architecture & Design.

‘After this, the design is usually given to a structural engineer for their input, and the level of detail on the drawings will increase to show compliance with building regulations. If you make big changes during or after this stage, there’s more work involved in altering the design, which will cost money.’

extension to Barnet home by N London Construction

The owner of this home in Barnet renovated to create plenty of space for visiting family. The ground-floor addition made room for a kitchen-diner that opened up to the garden. Completed by N London Construction, the extension cost around £48,000

5. Factor in the function of rooms

‘Choosing to create a living or dining room in the new space is the cheapest option’ says property expert Marta de Sousa. ‘If you are considering having a new kitchen, you’ll have the cost of fitting new pipes and wiring for the water, gas, and drainage, and the electrics needed for appliances, as well as the kitchen cabinets, worktops and splashbacks. With a bathroom, you’ll have to consider the added expense of tiling, the bathroom fittings, heating and running the large waste pipe through your floor.

6. Use salvaged or reclaimed materials

‘To save money without sacrificing quality, look for cast-off architectural pieces that fit your house’s period and style,’ says designer Charlotte Starling, of Velvet & Dash. ‘I find the most readily available are old doors. Their heavy, solid wood seems so substantial, plus the textures and finishes have a feeling that only comes with age.’

Cheadle Hulme single storey extension by Hende

When the owners of this house in Cheadle Hulme wanted a single-storey extension to add kitchen and dining space, they hired Hende, a building company with experience of the local planning process. The project cost £40,602 

7. Choose a simple shape, size and structure

‘Keep the extension footprint as close to a square as possible – this facilitates a clean roof design that avoids complex detailing, such as valleys and hips,’ says Ian Rock, director of surveying website rightsurvey. ‘An insulated flat roof, perhaps with fixed skylight, is the cheapest option. But it shouldn’t add greatly to the cost if you opt for a shallow lean-to roof clad with inexpensive concrete interlocking tiles, which offer good coverage and are quick and easy to lay.

‘Blockwork walls with an external render coating tend to be less expensive than brick and block, or timber frame. To minimise labour costs, position door and window openings so their edges correspond to the width and height of the blocks.

‘Building a larger extension can actually reap economies of scale. The roof and foundations are the priciest parts, so a two-storey extension can make a lot of sense because the infill wall construction is relatively inexpensive, even with the added cost of an upper floor.’

8. Be certain about what’s included

Get a fixed quote from your builder, and a very clear contract or letter that is signed by both parties to say what will be done for that money and what will be extra,’ says Kate Faulkner, managing director of independent advice site property checklists. ‘Consider who is responsible for signing off the building regulations with the local authority; if something needs to be redone, who pays? It should be the builder.’ Regardless of the insurance your builder has, make sure you have project insurance for the extension, as your normal household insurance won’t cover the work.

Extension to Yew Tree house by IQ glass

An addition to this listed cottage has given the owners a new dining room which merges
with the patio. Its glass box style means it is lit all day lone. The glazing was around
£34,000 from IQ Glass

9. Get clever with glazing

‘Use natural light and soft furnishings to enhance a simple extension,’ says architect Des Ewing, owner of Des Ewing Residential Architects. ‘Large spans of glazing can be very expensive – to keep the cost down, have less areas of glass and concentrate the natural light in specific places, such as the living area, where you will spend most of your time. Rooflights are a great way to bring natural light into the depths of room.’ And don’t forget to factor in artificial lighting to highlight key features and provide illumination.

10. Select materials with longevity for the interior

‘An all-white kitchen always looks fresh, while stainless steel is timeless and functional,’ says interior designer Amelia Carter. ‘A simple kitchen can be easily dressed up with colourful tiles, or beautiful work surfaces. Granite is a good for kitchen worktops. It’s a much harder surface than marble and not as porous, so lasts over time with fewer stains and water marks. For walls, glass tiles in bathrooms and kitchens are practical, and can be cheaper than ceramic or porcelain tiles.’

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