On Bulding an Extension

For personal reference, and in case it helps somebody else, here’s a summary of how we built our single-storey rear extension on our 1900 mid-terraced house in North London, completed March 2010 (some photos are here).

We’d not undertaken any building work before, other than having a replacement kitchen done a few years previously. Because of this, we proceeded with caution as you will see. What follows is a summary, but feel free to ask questions in the comments if you need more details.


We hired an architectural consultant to help us with the main issues, point out potential problems, and then to produce drawings and specifications for buildings regulations and the build itself. Hiring a consultant isn’t necessary, but we were new to all this, and it seemed generally wise to separate design from build. We paid him about £3,000 in total, and he worked with us until we hired the builder.

Planning & Regulations

While the design of the structure was within “permitted rights” (the amount you are allowed to extend a house before you need planning permission), nobody on our street had built a similar extension, so we also decided to apply for a “certificate of lawful use” for the land. This essentially confirms that according to the plans you have, the local council is happy to let you build something such as the one we had in mind. Again, this certificate isn’t necessary, but it was useful none the less to “test” the plans, and for peace of mind.

Because the house is relatively old, we also decided to pay for a structural engineer to draw up the plans and calculations for any issues relating to foundations and walls that might be affected. We added their drawings to the document set. This cost us £700.

Armed with all the drawings and specifications, we then decided on a “full plans application” to obtain buildings regulations approval before we started work. The more common route is to notify buildings control a few days before you start the build. The council then sends an inspector round a couple of times to make sure the builders are doing things “to regulations.” However, with the full plans route, we would know beforehand if anything in the plans (like footings, drainage or insulation) were not to regulation. This would avoid any nasty surprises from the council mid-way through the project. As it turned out, we never actually obtained the certificate before we started. The plans were rejected twice on minor points, but that didn’t matter as the main purpose of the exercise was to minimise the risk of any major issues, and to reassure us we had some decent quality plans and specifications in place.

The plans assumed we were to be moving a drain, so I rang our local water company to ask whether special permission was needed for that. They told me verbally that none was needed as we owned it. They never gave me that in writing, despite me trying to remind them.

While we were waiting for the plans application, I drew up a list of requirements for everything I could think of that the builder should do. This was something the consultant had started, but I added to it and turned it into my own work in the end. You can see this document here.

This written specification was extremely useful. In being as detailed as possible, it allowed us to get a fixed price quotation from the builder, who was also happy to sign a contract based on it (an FMB “small works” agreement). Note that the document describes what we wanted the builder to do, what not to do, and what materials we intended to buy ourselves. The aim was to remove as much confusion as possible.

Lastly, and in tandem with drawing up the written specifications, we served a party wall notice to each of our neighbours. While we had spoken to them earlier about our plans to build an extension (and showed them drawings in detail once we had them), this was further to avoid any issues once building had started. The Party Wall Act is designed to protect your neighbours from damage you do to walls they have part ownership of. It can be quite tricky if they object, so to ally their fears, we offered to take lots of photos of the exterior and interior of the parts of their houses where building work might affect them, and give them copies of these photos. In the event that building work produced damage (cracks, etc.), they would have “before and after” evidence which we could use to claim on insurance. Both neighbours signed their agreement without any issues being raised, although it did take me a while to understand (and for me to explain to them!) what was needed under the Act.

The Build

Once we were confident of specifications and plans, we found a good builder and got them to sign the agreement. The payment terms were agreed to be in four stages (foundations, walls, roof, remaining). We also agreed to withhold 5% of these payments, the outstanding total to be paid 6 months after completion. This was to ensure the builder would come back to do any “snagging” (leaking roofs, cracking plaster, etc.). Failure to do so would result in us not paying him this amount. The price he quoted against the specification was £30,000. He also asked for a deposit of £5,000 before starting work (the receipt for which we simply attached to the contract, since it didn’t appear to allow for deposits as part of the payment schedule). I also asked for sight of their insurance certificate prior to them starting.

Once work began, I maintained a “diary” of daily events in a spreadsheet. This had entries for every day, recording the date, whether there was anyone on site that day, how many hours of work were done, what the weather was like, and any notes. This was to use in the event of any dispute over what had been done when.

The build then proceeded with very few issues. There were times when I realised the builders had not done exactly what was given in the specification, but I was willing to let that go. We now have an extension that has been standing well and without any problems other than an initially leaking roof due to a mistake with some leading. The builders swiftly fixed this a month or so after completion.

Overall result: success. I don’t think I would have done much of anything differently with hindsight.