How to Plan a Home Extension (So You Don't Learn the Expensive Way)

This article is adapted from Chapter 2 of Your Seven Step Home Extension Plan by Ashton Paul.

Some people seriously misjudge what it means to have builders in their home for 10 to 12 weeks. Without a plan, you are completely in the hands of the builder — and that is not the place to be. A good plan is the difference between success and failure; even an OK plan beats no plan at all.

Don't learn as you go

Why pay the price of your own mistakes when you can learn from other people's? "Learning as you go" on a building project is genuinely expensive: unprepared projects routinely overspend by 25%, and overspends of up to 100% happen. One friend of the firm, one week into a sixteen-week project: "I wish we didn't start this project!"

The traditional alternative — employing an architect or surveyor to design, obtain approvals and project-manage — works, but understand the economics: typically 10–15% of build cost, and on smaller projects those fees eat a meaningful share of the total budget before a brick is laid. That's exactly why knowing the process yourself matters, whichever route you take. (We've written about when you actually need an architect — and it's also the gap cowboy builders rely on homeowners not filling.)

The realistic timeline

Here's the arithmetic most people get wrong:

• Drawings: 2–3 weeks to prepare, plus revision rounds
• Planning application: 8 weeks statutory period — longer when the department is busy, and everyone wants to build in summer
• Then tendering, contracts and builder lead times

If you haven't allowed three months, you're under pressure from the start. And pressure means fast-tracking — and speeding up a project always costs, in money or quality. It's like a space-saver staircase: start on the wrong foot and you're going nowhere fast. Focus on the completion date, not the start date.

Delayed design: the builder's profit margin

This is the sharpest lesson in the chapter. Deferring design decisions to "agree on site" is delayed design — and it's the death knell of a budget.

Some larger contractors deliberately take on projects at cost, knowing the undesigned elements in the specification will generate variations and additional works priced at much higher margins. Paul has seen a contract sum increase by 50% — £250,000 of additional works — through exactly this mechanism. Even at zero margin on the original price, a 25% margin on the variations nets the contractor a healthy profit across the whole job.

The defence is simple: demand a full design before building works start. Limit provisional sums and provisional quantities to the genuinely unknowable. If you have a vision of the space, ask for 3D models or sketches until you understand what's proposed — it's your house; they can only say no.

Prepare, prepare, prepare

How much preparation would you do to avoid overspending by 25%? A realistic build plan sets out the key stages, tells you when to seek professional advice, includes cost and time contingencies, and sets the parameters of what's reasonable — so the builder and his subcontractors can't take advantage of you.

Not everything will go to plan. That's not the point. The point is that planning makes you aware of the journey before you're on it.

The full plan — all seven steps, with checklists — is in Your Seven Step Home Extension Plan (£12.99). Or start with the numbers: our guides to extension costs in Tandridge and getting comparable builder quotes, our home extension service, or a free consultation on your own project.