Snagging a New Extension: Defects, Retentions and Call-Backs

This article is adapted from Chapter 10 of Your Seven Step Home Extension Plan by Ashton Paul — the book that shows homeowners how to manage a project and avoid the stresses and strains of the home extension process.

Ever wondered what happens when the builders finish, the vans disappear — and then the hairline cracks start arriving? Or worse: the builder stops answering your calls? It does happen. The good news is that the industry solved this problem long ago, with two mechanisms every homeowner should insist on: the defects period and the retention. Together, they are your snagging list's teeth.

Settlement: which snags are normal

First, calibrate your expectations. Following any new building works there is a period of settlement: minor cracks appear in new plaster, and the new working parts — doors, windows, heating, drainage — get their first full run. Whether you build in summer or winter, the change in temperature through the following months will test the works. A hairline crack over a new opening in month three isn't a scandal; it's physics.

That's precisely why building contracts provide a rectification or defects period of around six months after completion. The point isn't that defects are unexpected — it's that they're expected, and the contract keeps the builder on the hook to return and put them right once the building has had its shake-down.

So a sensible snagging process looks like this: note everything you find as you find it (your project diary is still working for you here), report items in writing as they arise, and hold the formal review at the end of the defects period — when settlement has finished manifesting — before any final money moves.

The retention: your snagging list's teeth

Here's the arrangement, exactly as the industry has tried and tested it: 5% is retained from every valuation during the works, reducing to 2.5% when the job is completed — and that 2.5% is held for the defects period, usually six months (the book's checklist says three months as a minimum), before being released.

Paul won't attempt to justify the percentage beyond this: it works. In 28 years he never had a problem with it. A client once challenged the 2.5% as too little — but in practice it is just enough money to get the builders back, or to pay somebody else to resolve any issues if they won't come. If no defects are detected, the retention is released and everyone parts friends.

The corollary is stark: if you've already paid 100%, your snagging list is a wish list. With no retention, you're relying on the builder's diary and goodwill against every other job in his book. This is why the retention belongs in the contract from day one — it's part of the payment structure we set out in the building contract post, agreed before anyone priced anything, not a deduction invented at the end (which is a dispute, not a retention).

Finishes and fittings: where post-completion budgets go to die

Chapter 10 spends surprising time on finishes — because the end of a project is where they ambush people, in two directions.

Direction one: the specification grew quietly. Paul tells the story of a project he specified where the client's brief began with "simple tastes — a standard 600mm Neff cooker". By tender time, the design had gathered an 1800mm super-oven with extractor and a cast-iron freestanding Victorian bath with ornate taps, and the tender came back at £120,000 plus VAT — over budget, with roughly a 20% cost increase attributable to the standard of fittings and finishes alone. In a modest family home you can buy and install a kitchen for £10,000 or £40,000; the standard of your finishes and fittings will absolutely control your budget.

Direction two: the costs that were never in the contract. Carpets, curtains, shutters, furniture, the £1,000 chandelier you fall for while shopping — none of it is in the building contract, and all of it lands right at the end. Keep a separate pot for it, and if something like that chandelier needs specialist fixing to the ceiling, tell the builder in advance. (If you've pleaded poverty all project and then produce statement lighting, expect some ill will from trades who felt the squeeze — price pressure travels down the contractual line.)

The planning-stage version of this lesson lives in our cost control guide: only about 70% of your budget buys building work. The completion-stage version: make sure there's still money in the other 30% when you get here.

Finishing it yourself: the legitimate shortcut

If the budget is tight at the end — or from the start — Chapter 10 offers a strategy Paul used on his own house: get the builder in for the difficult, dirty stuff, and finish the pleasant bits yourself.

He and his wife took on the painting, some paper-hanging and simple joinery at the end of their own extension. The saving was real: two weeks of three men at £150 a day — £4,500. The honesty is real too: it took them three times longer than the trades would have taken, and a couple of doors remain unpainted to this day. DIY is a cost control measure where quality stays literally in your hands — but time is the variable you surrender.

A stronger version is the shell finish: the builder does the demolition and construction, gets paid, waves goodbye — and individual tradesmen help you complete the finishes. A client of Paul's saved £750 on decorating costs this way, and self-employed decorators are often better and cheaper than a builder's own team (and frequently not VAT-registered, which helps again).

One hard limit: don't be tempted to run the whole project with individual trades and no builder. Unless you have exceptional project management skills and real knowledge of the building industry, removing that layer of planning and organisation multiplies every risk in this series. To be clear, as the book is: Paul does not recommend it.

Keep your eye on the completion date — to the very end

The thread running through the whole chapter: it was never about the start date; it's about the completion date. That discipline matters most at the finish, when you're tired and inclined to wave things through.

The cautionary tale is from three new houses Paul ran in Dorking: a six-month contract that took nine, with the killer delay caused by a £2,200 gas installation fee. The cheque had gone missing while the contractor was on holiday; Paul advised the client to simply pay the fee directly and offset it later, at a cost of about £600 in VAT. The client declined — and waiting for the contractor's replacement cheque cost three months of the gas supply, which on three rental houses at £1,000 a month meant £9,000 of lost income. Versus £600. The client's eye was on a line item; the target was completion. (The contractor's £2,000-a-month claim for those extra months on site, incidentally, was substantiated down to a fraction at the final account — the paperwork earning its keep one last time.)

Your extension won't hinge on a gas main — but the principle scales down perfectly: at the end of a project, small economies that delay completion are usually false ones.

The post-completion checklist

From the end of Chapter 10:

• Hold the agreed 2.5% retention for at least three months — ideally the full six-month defects period
• Be realistic about your finishes, fittings and furniture budget — the quality of your finishes can blast your budgeting
• Focus on the completion date, not the start date
• Pre-order anything involving the utility companies — they're the delays you can't control
• And note everything down as settlement reveals it — then hold one proper review

Post-contract matters are step seven of the seven — the step most homeowners never knew existed until they needed it. It's built into every project our surveyor-led home extension service runs: retention held, defects period observed, final account reviewed. The complete process is in Your Seven Step Home Extension Plan (£12.99).