An extension is the most practical way to add floor space without moving: an extra kitchen-diner, another bedroom, a workshop against the back wall. Behind every good project sits a chain of decisions to be made in the right order. Get the sequence wrong and you’ll redo work later. Here are the four stages almost any UK extension passes through, from planning to final plaster.
Step 1: Designing the Layout and Securing Permissions
Start with two parallel questions: what you want, and what your local planning department will sign off on. Under permitted development, a single-storey rear extension can project up to 3m from the back wall for terraced and semi-detached homes, or up to 4m for detached, provided the build sits under 4m in height and doesn’t rise above the existing ridge. The Larger Home Extension Scheme stretches that to 6m and 8m respectively, but it needs prior approval from the council.
Anything beyond that envelope needs a full planning application. Houses in a conservation area, listed buildings, and properties inside a national park usually have their permitted development rights restricted, so it’s worth checking the position before any design work starts. A good architect or an experienced design-and-build contractor will translate’I want more light at the back’ into something that fits those rules and the Building Regulations.
Step 2: Groundwork and Upgrading Your Water Mains
Once the plan is locked, the digger and pipework take over. Foundation depth and the type of footing are worked out by a structural engineer. For most domestic extensions, that means traditional strip footings, with raft slabs or mini-piles only on tricky ground. With the trenches open, it’s a good moment to look at the water supply. The original mains pipe is often undersized, in lead, or in old steel, and simply not built to feed a new kitchen and a second bathroom.
The standard for a new domestic supply is 25mm blue MDPE pipe, WRAS-approved and made to BS EN 12201. It handles up to 12.5 bar and won’t corrode. The pipe needs to be buried between 750mm and 1350mm deep, measured from the crown, on a sand bed, with at least 350mm of separation from any gas or electrical services. Most jobs use coils or six-metre lengths, depending on the run length and site geometry. If you or your contractor want to see what’s available, the range is here: https://www.monsterplumb.co.uk/pipe-amp-fittings/pipe-tube/mdpe-pipe. The connection to the main beyond the boundary stays with the water authority, but the run from the stop tap to the house is yours and your plumber’s.
Step 3: Structural Integrity and Insulation
The frame, openings in load-bearing walls, and the way the extension ties into the house all sit under Part A of the Building Regulations. Building control will want a calculation for every steel and spanning member. Don’t cut corners on the structural engineer. Redoing foundations after the walls are up costs many times more than the drawing did at the start.
Thermal performance falls under Part L. For extensions, that means hitting target U-values: a typical new build-up with a filled cavity of around 150mm lands at about 0.18 W/m²K, roofs at 0.16 W/m²K or lower, and windows at 1.4 W/m²K. In practice, that drives the cavity width, the insulation you choose (PIR, mineral wool, EPS) and how cleanly it goes in. Even the best material loses a third of its performance once air gaps and thermal bridging creep in around reveals and lintels. Ask your building control inspector early what they want to see at sign-off. Going back into finished walls always costs more.
Step 4: The Final Polish: Why Professional Plastering Matters
By the time the shell is up and the roof is closed in, the plastering stage is the last real chance to hide the bumps and leave clean, flat surfaces ready for paint, tile or paper. This is where a good local plasterer earns their fee: float and set onto a mineral substrate, or a skim coat over plasterboard. Both call for a feel for the material and a kind of timing you can’t pick up from a weekend of YouTube tutorials.
A solid plasterer keeps the mix consistent, the timing between coats right, and the drying conditions in check. Fresh plaster gives off moisture for four to six weeks. Paint too soon and the bubbles and patches will give it away the moment your first guest walks in. Cracks around lintels, hollow spots in finished walls, corners that aren’t quite square: those are the signs of skimping on the plasterer. If you’re going to live with these walls for years, the gap between ‘good enough’ and ‘mirror-flat’ pays itself back every time your eye runs across the room.