How Do I: Design, Build & Cost My Extension?

The property mad crew over at Real Homes asked me to pop together a short guide on some of the different options available to would-be house extenders with regard to design, build and management. We’re slap bang in the middle of our own build, which I’m project managing, but we’ve also had some architect assistance at the outset. There are a myriad of different ways to approach extensions and there’s definitely no one-size-fits-all solution but here’s a starter for ten to get you going. Having just spend a few days last month looking round some competition shortlisted extensions, I can attest that us Brits are a talented lot…. so read on and go for it!2016-10-20_0001Combining the trend for open plan living with increased moving house costs and the relaxing of planning rules means adding single storey extensions has never been more attractive for homeowners.

Here’s a short guide to options on who could build it for you.


Creating a jaw dropping and structurally complex extension takes skill and experience. For that it’s best to employ the services of an architect to design & draw the scheme, navigate the planning process and create technical specifications for builders.

Fees from large glossy firms can be high so shop around for talented local practices or single practitioners offering fixed rates for smaller residential contracts. The pay scale is fixed so you’ll know costings up front to help plan your budget and you have the peace of mind that a professional is to hand. On more complicated projects it’s likely you’ll get a superior design finish with an architect in charge.

Find one at, on RIBA or ask friends locally.


For a simpler extension, you might feel that you don’t need or can’t afford full architectural input. Maybe consider using an architectural technician to create drawings, then a good interior designer to organise the builder, specify layouts /products, and trouble shoot if required. Remember that your builder will want instructions on where you want electrics and plumbing, not easy if you’ve never done it before.

Make sure you agree and document all that you will do and all that they will do up front, to avoid problems later. Interior designers often have preferred builders who they know and trust, plus get great rates on products which they pass on to you, so their fees can be well worth paying.


For small & very straightforward extensions there’s nothing to stop an enthusiastic amateur drawing up scale plans and submitting them to local planners to get a good steer of what would be acceptable and permitted. If your budget is tight a decent local design & build company will be able to cost up and see the job through to completion, with you or they organising the required Building Notice and visits for compliance with Building Regulations.

Word of mouth recommendations are best but if not try the Federation of Master Builders / other reputable trade associations. Get 3-5 quotes, and make sure you have a comprehensive list of what you want them to do when they visit. There’s no buffer zone of an architect here, so you need to trust and like your builder, as project manager you may be working with them for months!

Be confident that the work is up to scratch, as any work carried out not to Building Regulations may have to be rectified even after completion, so mistakes can be costly. Stage payments by BACS or cheque should be pre-agreed and a proper contract drawn up with details of start & end dates, cost of materials and labour, working hours, extent of works and responsibilities.

For works done under PD, consider getting a Certificate of Lawful Development for when you sell.


Build costs – £1200 to £2200 per sq m, dependent on technical specification and quality of finish

Professional Fees : Approx 10-15% of the build cost. To include drawings, planning application, building regs, structural engineer, architect / designer. This may be higher if your professional has passed on their special labour and material rates for you, but you’ve saved on those so it balances out.

Bi-Fold Doors – £1000 – £1600 per m width

Kitchen – £6000 – £20,000 dependent on specification, inclusive of appliances

Flooring – £20-80 per sq m dependent on specification


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