The ingredients

The great thing about designing your own building is that you can start rounding up materials before you have any idea what it's going to look like. The first thing we got was some reclaimed timber, previously the seating of a local bar, until they decided to rip it all out and replace it. Some of this later turned into work benches.

Windows and doors are parts of a build that have the potential to get very spendy, very quickly. Having obtained a quote for a set of bifold doors that was nearly as much as we were planning to spend on the entire build, we started scouring classifieds websites for something a bit more careworn and affordable. An advert in Gumtree led us to a massive set of sliding patio doors. Yes, they had glazing too. The cost was a couple of hundred quid, compared to £500 for a new set, and £1,000 or more for bifolds.

On to the windows. You can actually buy custom-made double glazing units fairly cheaply, but we wanted to do this even cheaper if possible. A local reclamation yard provided two double glazed windows in the same "trouser press brown" finish. No idea what the U-value of these is, and they've seen better days, but they're still a vast improvement on the single glazed windows that feature on the bigger sizes of garden shed.

For the timber frame, we decided to use standard construction timber (or "carcassing" to give its thrash metal-tastic alternative name). This comes in standard sizes, including 44x100mm and 44x150mm. In proper houses, 100mm is usually reserved for non-load bearing walls, but we decided to use it for the walls and floor, to make the frame easier to handle and keep the overall height of the structure low. An initial plan to use 100mm timber for the roof was quickly beefed up to 150mm after Celia's dad made some remarks about the unsupported span, and reminded us that we'd probably need to clamber about on it to fit the roofing.

This building is going to be used all year round, so some insulation is a must. Quite a lot of insulation in fact, given its size. For value and thermal performance Celotex board is hard to beat, but it's messy to cut to size, and has that landfill-in-waiting feel about it. It felt right to blow some of the money we were saving elsewhere on some insulation with good performance and green credentials. so we want for Steicoflex insulation, which is made from recycled wood fibre and is nicer to handle than fiberglass, performs better than wool, and will hopefully return to the earth with good grace one day, instead of hanging around forever. It's available in standard widths to suit various joist spacings.

We decided to sheath the outside of our frame with insulating board, which reduces heat transfer through the timber structure, stiffens up the frame, and also acts as a temporary weatherproof skin for the building until the waterproof membrane and cladding are added. Also, if you're a cat, it's great for sharpening your claws on.

For the lining of the interior of the building we chose plywood, taking the cheap and cheerful option at our local DIY megashed. With hindsight I wish we'd gone for proper wall cladding material, or at least better quality ply. The price was right, but the amount of warped and damaged pieces became very tiresome later on. We used OSB (the board that looks like a thin sheet of Weetabix) for the floor, with the idea of sanding and varnishing this later.

We didn't decide on roofing or cladding material until after the build had started. For the roof, we wanted something that would work at a shallow pitch, and be removable if we ever dismantled the room. The angle of the roof ruled out rustic materials like shingles, and I didn't like the idea of having to replace a huge expanse of felt every few years, so in the end we went with box profile sheet metal.

We were told by our local supplier to pick a light colour, because otherwise the room would quickly become unbearably hot in the sunshine. However some quick research revealed that if you've got a decent amount of insulation, the colour of your roof becomes much less important. We ended up going with a dark graphite colour to make the building blend in a bit.

For the exterior cladding of the walls, we had a big supply of featherboard from the fence around the site. These weren't in the best condition but we sanded them up and treated them with a fantastically expensive fire proofing solution (you're supposed to build the exterior walls of outbuildings near property boundaries from "substantially non-combustible materials", which seems like common sense if you're constructing a large wooden box) before applying a wax-rich grey paint.

With these materials rounded up, the next stage was to finish preparing the site.