[ Design tips ]

Robert Houmøller, co-founder of architectural firm Merrett Houmøller, offers advice on how to get the most out of your extension

* An average-sized extension costs around £2,500-£3,000sq m. Ground-floor extensions usually cost more than roof or loft extensions, with basement projects the most expensive. If you’re looking to increase floor space, the loft often gives the most value for money. Plus, remember that around 10-15 per cent of the construction cost should be set aside for planning and architects’ fees.

* Don’trestrict yourself to architects in your area – they’re often happy to travel for interesting projects. The most important thing is finding someone you get on with and who understands how you live your life. Write a brief, with examples of designs you like. Architects will pick up on all sorts of cues, and this can point them in the right direction.

* With builders, recommendations for local businesses can be useful. Ask your neighbours, then tender to at least three contractors. Arrange to meet them and get a feel for how they operate before you make a choice.

* Rather than drawing a rectangle extension and filling it with furniture, start by looking at your home and garden in its entirety. This will help you to see how a potential extension could affect existing spaces and design it to meet your individual needs.

* Utilities are easy to install during building work, but much harder to fit afterwards. Underfloor heating may only cost a few hundred pounds and take an afternoon to install while laying the floor screed, but it’s impossible to do once that’s been completed. Similarly, it often makes sense to upgrade your boiler during a build, as plumbers are already on site. Spending a little extra in the short term will save money in the long run.

* You can usually build higher and further out under permitted development rights, but you will be restricted to using materials that match the existing house. If you submit a planning application, the rules are less strict, so you could get consent for a timber-clad extension to a brick house.