Start with a simple shape

The single biggest driver of extension cost is not the quality of the finishes — it is the geometry of the building. A plain rectangle with a pitched or flat roof is, square metre for square metre, the cheapest thing you can build. Every corner you add means extra foundations, extra brickwork, extra roof, extra guttering and extra labour, and those costs multiply rather than add up.

An L-shaped or wraparound extension can easily cost 20–30 per cent more per square metre than a straightforward box of the same floor area. So before you fall in love with a design, ask your architect or designer whether there is a simpler version that gets you the same living space. A slightly deeper single-storey rear extension almost always works out cheaper than a shallower one that has to turn a corner.

It also pays to work with the existing structure rather than fighting it. Extending off an existing wall, tying into the current roof line and avoiding any need to move the kitchen or bathroom plumbing will shave thousands off the quote. Ask yourself what you actually need — a bigger kitchen-diner, a downstairs bedroom, a utility — and design around that requirement rather than around a picture you saw online.

Reuse openings, services and materials

New openings are expensive. Cutting a structural opening for a set of bi-fold doors means a lintel, temporary propping, a steel beam and possibly a Building Regulations application for the structural work. If you can, position the extension so that an existing back door or window becomes an internal opening instead. You save on the steel, the labour and the making good.

Services are the hidden budget killer. Moving a soil stack, re-routing drainage, relocating a boiler or shifting a consumer unit can run into several thousand pounds, and none of it is visible when the job is finished. Keep the new space on the same side of the house as your existing drains and make sure your builder or plumber walks the route before contracts are signed. A quick CCTV survey of the drains is money well spent compared with discovering a problem halfway through the groundworks.

• Reuse existing door and window openings wherever the layout allows.
• Keep new sanitaryware close to existing waste pipes.
• Salvage bricks, roof tiles and paving from the demolition for reuse elsewhere on the plot.
• Retain mature planting and boundary walls — removing and reinstating them costs more than you'd think.

Nail the paperwork before anyone lifts a spade

Rework is the most expensive thing in construction. If a design changes after the foundations are in, you pay twice — once for what was built and again for what replaces it. Spend the time upfront on drawings, a structural engineer's calculations and a proper specification, and get any planning permission or lawful development certificate sorted before you start.

Most single-storey rear extensions fall under permitted development, but permitted development rights vary between councils and can be removed by an Article 4 Direction. A lawful development certificate costs relatively little and protects you when you come to sell. Building Regulations approval, meanwhile, is not optional, and a good builder will want the plans before pricing the job.

It is also worth telling your neighbours what is happening and when. A party wall agreement, where it applies, is far cheaper than a dispute, and a bit of goodwill keeps the site running smoothly.

Buy materials at the right moment

Material prices move. Timber, steel, glazing and insulation all swing with supply, demand and the time of year. Ordering early — but not so early that you have nowhere to store things — lets you lock in a price and avoid paying a premium when a merchant is short of stock.

Practical steps that genuinely work:

• Ask for a fixed-price quote for the big-ticket items: windows and doors, the kitchen, roof lights, structural steel. Glazing in particular can be ordered months ahead.
• Buy at quieter times of year. Late autumn and winter are often better for grabbing a deal on outside works than a busy spring.
• Set up trade accounts at local builders' merchants — the discount on a full extension's worth of blocks, timber and insulation adds up fast.
• Check for ex-display kitchens, end-of-line tiles and reclaimed bricks before buying new. On a project this size, seconds and ex-display stock can save thousands.
• Accept reasonable delivery lead times instead of paying for next-day service on every order.

Keep the specification sensible

It is tempting to specify the very best of everything, but the returns rarely justify it. Spend money where it is hard to change later — insulation, underfloor heating pipes, drainage, structural openings and roof quality — and be more relaxed about things you can upgrade in a few years, such as light fittings, paint and even the kitchen worktop.

Higher insulation and good glazing reduce running costs and are a Building Regulations requirement anyway, so don't treat them as optional extras. On the other hand, an eye-wateringly expensive bespoke front door or a designer bathroom suite can wait until the budget has recovered.

Finally, consider how you will run the project. Acting as your own project manager can save the main contractor's overhead and profit, but only if you have the time, patience and a reliable set of trades lined up. For most homeowners, the safer saving is to appoint one good builder who can do everything, agree a clear schedule of works, and hold back a retention of around 5 per cent until snagging is done. That single clause has saved more homeowners more money than any discount on bricks ever will.