How did a £25,000 kitchen allowance become £75,000?

Because it was a provisional sum — an allowance in the contract for a kitchen not yet designed, to be omitted later and replaced with the actual cost. On the North London extension in Your Seven Step Home Extension Plan, the client chose a £50,000 Italian kitchen and £25,000 of appliances, with £7,500 of specialist installation on top. The lesson the book draws: design and specify the fixtures and finishes before tender, so there is little left to decide later at variation prices.

Why do extensions go over budget even with a good builder?

On the North London extension in this case study, the builder won a competitive tender against a robust specification and delivered excellent construction and site management — and the project still finished at treble the budget. The majority of the extra cost came from client changes made during the build. Cost control starts with the brief and the design, not with the builder.

How do I stop changes wrecking my extension budget?

Three defences: make sure everyone whose opinion counts has agreed the brief and budget before work starts; design and specify as much as possible up front so there is little left to change; and use 3D visuals or walk the positions on site before installation, so you aren't relying on clients reading technical 2D drawings. Late changes — after installation — are the most expensive kind.

How much does a home extension cost per square metre?

As a current guide, extension build costs typically run at £2,500–£3,500 per square metre depending on specification and complexity, with professional fees on top — typically 10–12%. Note that the North London project in this case study dates from 2005/6, so its figures reflect prices of that era rather than today's rates.

Case Study: How Client Changes Trebled a £250,000 Extension Budget

Ashton opens this story in Your Seven Step Home Extension Plan with an unusual admission: Mika was — and remains — his favourite client of all time, and they're still good friends. That matters, because this case study is about a project that went badly over budget without a villain anywhere in sight. No rogue builder, no defective work, no dispute. Just a budget of £250,000 that became a final cost of £750,000.

He also explains why he prefers recounting projects that didn't go to plan: "I'm self-critical and always blame myself first" — and "you learn from your mistakes." In that spirit, the book's heading for this one isn't "where did they go wrong?" but "where did we go wrong?"

The project

Mika was in her late thirties, ran a small fashion company designing and selling upmarket ladies' clothes, and was married to a lawyer who ran his own firm. They had two children and lived in a very nice three-storey extended home in North London. The plan: demolish a small side extension/garage and construct a three-storey extension, with a budget of £250,000. This was back in 2005/6 — so read every figure in this story at that era's prices, not today's rates.

On paper, the setup was close to textbook:

Drawings prepared, planning and building control approvals obtained
A specification drafted in line with the husband's brief — budget £250,000, kitchen £25,000 including appliances
Contract documents drafted using the NBS form of specification — very robust; Ashton still rates it among his best
The job submitted for competitive tender, won by a well-regarded high-end London building company

And the builder delivered: great construction, great site management. If you've read our other case studies — like the Earls Court refurbishment with no contract at all — you'll recognise that this project had every safeguard that one lacked. But, in the book's words: oh boy, did this project go wrong...

What happened

Two details from the background turned out to matter. Mika's husband — the one who set the budget — specialised in property litigation. And Mika, a designer by trade with great taste and a strong personality, was used to getting her own way.

The specification allowed £25,000 for the kitchen and appliances. Mika had no intention of spending £25,000 on a kitchen. She spent:

£50,000 on an Italian kitchen — one of only three of its kind in Europe, including the showroom
£25,000 on the appliances
£7,500 on recommended specialist installation

To be fair to her, she was ahead of trend — the natural, light grey tones she chose are still in fashion today. But the kitchen was only the headline. Change after change flowed into the project, and the cost reporting told the story in real time: the projections climbed from £40,000 over budget to £80,000 over, and by month three an emergency meeting was called with the projection at an alarming £120,000 over — mostly due to client changes. Ashton notes he seemed more concerned than the clients were.

At the end of the project, the majority of the extra cost was down to client changes. Final cost: £750,000 — treble the budget.

Where did we go wrong?

The book's self-critique lands on three failures — and every one of them is a design-stage failure, not a construction one.

Client briefing. The husband set the budget; Mika had other ideas. The lesson learned: make sure the clients are on the same page about the proposed works before the project starts. A brief agreed by one decision-maker in a two-decision-maker household isn't a brief — it's a placeholder for future changes. (Ashton's tongue-in-cheek policy: side with the wife and declare the bias at the first opportunity.) If you're at this stage now, our guide to planning a home extension starts exactly here — with the brief, before the drawings.
Detailed design — or what the book calls "lazy design". The £25,000 kitchen figure was a provisional sum: an allowance for something not yet designed, to be omitted later and replaced with the actual cost. The actual cost was £75,000. The lesson: design and specify as many of the fixtures and finishes as possible before tender. The less room there is for variations or changes, the less ability your builder has to increase the price — and the less temptation there is to "decide later", because later is always more expensive. It's the reason our kitchen renovation work pins down the kitchen itself — units, appliances, installation — as part of the design, not as a number in a contract.
Two-dimensional drawings. On completion of the electrical installation to the master bedroom suite — her bathroom and dressing room, his shower room and dressing room — Mika asked for the lighting and power points to be changed. The power and lighting drawings had been completed six months earlier, and instructions were in place to get approval for all extras from her husband; it was far too late to change them cheaply. The lesson: not all clients can read technical drawings — and it's the designer's job to know that. 3D visuals may be required, or simply revisiting the positions on site with the client before installation. Ten minutes walking the room with masking tape costs nothing; moving sockets after second fix costs plenty. This is a large part of why we treat interior design — lighting, power, finishes, fittings — as part of the project design rather than an afterthought.

What this project teaches

The instinct after a story like this is to blame the client. The book pointedly refuses to — and that's the real lesson. Changes are what clients do when the design hasn't finished asking them questions. Every one of Mika's expensive decisions — the kitchen, the lighting, the finishes — could have been made before tender, when they would have been priced competitively, or at least confronted honestly against the budget. Instead they were made during the build, one variation at a time, at variation prices.

Notice also what didn't fail. The specification was robust, the tender was competitive, the approvals were in place, the cost reporting caught the overruns early and reported them honestly, and the builder did excellent work. Process didn't prevent the overspend — but it meant every pound of it was visible, explained and chosen, rather than discovered at the end. There's a world of difference between an expensive project and an out-of-control one.

Ashton writes that his current project management approach was designed in large part around the lessons of this project. The full story — including the chapters on cost control and being a good client that grew out of it — is in Your Seven Step Home Extension Plan.

Planning an extension with strong opinions in the house?

If you're planning a home extension and you already suspect the budget and the wishlist don't agree, that's not a problem to hide — it's the first thing to design around. Book a free consultation and we'll help you get the brief, the budget and the specification telling the same story before a builder ever prices it.