Six Costly Build Mistakes

Introduction

Before you build

Building or extending a home is a big undertaking, full of decisions, money and moving parts.

Done well, it's calm and rewarding. Done in a hurry, it tends to mean budget creep, delays and stress. The good news is that the most expensive mistakes are also the most avoidable. Here are six we see often, and how to stay clear of them.

01

Starting without a clear brief and design

Diving into a build before you've resolved what you actually want — on paper — is where budgets unravel.

02

Leaving the big decisions too late

Kitchens, bathrooms, finishes, structural choices: left late, they hold up the build and invite costly, last-minute changes. Pin down the big things early, so the build runs to a plan rather than a guess.

03

Choosing a builder on price alone

The cheapest quote is rarely the cheapest build. Compare like-for-like, look at recent work, take up references, and value communication and reliability as much as the number at the bottom. A good builder is worth paying for.

04

Skipping proper drawings and Building Regs

Vague drawings mean the builder prices assumptions and fills the gaps on site — usually at your cost. Clear, coordinated drawings and a proper Building Regulations package keep everyone building the same thing, and keep surprises off the invoice.

05

Trying to manage it all yourself

Coordinating designers, engineers, planners, building control, suppliers and trades is a job in itself. Without someone holding it together, things slip through the cracks. This is the team we help pull together and steer:

06

Believing the 'eco' sales pitch

'Eco' and 'passive' get used loosely. Real performance comes from good fabric, careful detailing and airtightness — not a badge on a product. Be curious, ask for evidence, and design performance in from the start.

Fast facts

Worth knowing before you start

Hover over or tap a card for more detail.

Add 10–15% contingency

Most overruns come from late changes on site and unforeseen ground conditions.

A contingency is money set aside in your budget but not allocated to anything, to cover the unexpected: poor ground found when digging foundations, hidden problems in an old building, or changes you decide to make along the way. Older buildings and tricky sites sit towards the top of the range. If it isn't needed, it's a welcome saving at the end.

New-builds are VAT-free

A new home is zero-rated; self-builders reclaim VAT via HMRC's DIY Housebuilder Scheme.

Building a brand-new home is zero-rated for VAT, so a VAT-registered builder shouldn't charge VAT on the construction. If you're self-building and buying materials yourself, you can reclaim most of that VAT through HMRC's DIY Housebuilders Scheme once the home is finished, within three months of completion. Extensions and alterations are normally charged at the full 20%, though some conversions qualify for a reduced 5% rate.

Party Wall Act 1996

Two months' notice for work to a shared wall; one month to build on the boundary or dig near a neighbour's foundations.

The Party Wall etc. Act protects you and your neighbours when work affects a shared wall, the boundary, or the ground close to a neighbour's foundations. You serve a formal notice; your neighbour can agree, or appoint a party wall surveyor, whose fees you normally pay. It's separate from planning and Building Regulations, and serving notice early avoids delays to your start date.

Vet your builder

Insured, VAT-registered where relevant, with recent references you can visit.

Ask to see their insurance certificates, visit a recent job and speak to that client, and check they're happy to work to a written contract with a clear price and programme. Membership of a trade body is a good sign but not a guarantee. Be wary of large payments up front; staged payments for work completed are normal.

Before you build

Six questions worth asking

• Do we have a clear, agreed design before we ask anyone to price it?
• Are the big decisions made — or at least programmed in?
• Have we compared builders on like-for-like information?
• Do we have coordinated drawings and a Building Regs package?
• Who is holding the whole project together and protecting the design?
• Where do the performance claims actually come from?

This guide is general information for projects in England and is not legal, planning or design advice. Every site is different — always check current rules with the relevant authority, or ask us.