7 Extension Mistakes That Cost Homeowners Thousands

After years of building extensions — and occasionally being called in to fix other people's — I've seen the same mistakes come up again and again. Some cost a few hundred to put right. Others cost tens of thousands. Here are seven that I'd want to know about if I were extending my own home.

1. Not Getting a Proper Survey Before You Start

Designing an extension without a measured survey is like tailoring a suit without taking measurements. An architect working from estate agent floor plans or rough dimensions will produce drawings that don't quite match reality — and those discrepancies become expensive problems on site.

A measured survey costs £300–£600 and gives your architect exact dimensions, levels, and existing structure details to work from. A drainage survey (£200–£400) tells you where underground drains run, which directly affects where you can put foundations. Skipping these to save money upfront almost always costs more later.

2. Choosing a Builder on Price Alone

The cheapest quote is rarely the best value. I've lost count of the times I've been asked to quote on finishing or fixing a job that another builder started and either abandoned or botched. The homeowner saved £5,000 on the original quote and spent £15,000 putting it right.

Get three quotes, check references, visit a completed job if possible, and make sure your builder has proper insurance (public liability and employer's liability at minimum). A builder who is transparent, communicates well, and has a track record is worth paying more for.

3. Underestimating the Total Budget

The build cost is not the total cost. On top of the builder's quote, you need to budget for:

• Architect/designer fees: £1,500–£4,000

• Structural engineer: £500–£1,000

• Planning application: £250

• Building regulations: £400–£800

• Party wall surveyor: £1,000–£1,500 per neighbour

• Kitchen/bathroom (if not included in the build quote)

• Flooring, decoration, and furnishing

• Contingency: 10–15% of total

On a £50,000 build, the true total is often £60,000–£70,000 when everything is included. Know this number before you commit.

4. Forgetting About the Knock-Through

You've built a beautiful new extension, but it connects to the existing house through a doorway-sized opening. The extension feels like a separate room, not an extension of your living space. This happens when homeowners don't plan (or budget) for opening up the back wall properly.

If the wall between your house and the extension is load-bearing (which external walls always are), you'll need an RSJ (steel beam) to span the opening. A wide opening with a properly sized RSJ transforms how the extension feels. Budget £1,500–£3,000 for the steel and installation, and discuss the opening width with your architect early in the design.

5. Neglecting Natural Light

A deep single-storey extension can make the room it's attached to darker. You're moving the back wall further from the existing windows, and the new roof blocks light from above. I've seen extensions that looked fantastic on paper but created a dark, gloomy space in practice.

Solutions to consider at the design stage:

• Roof lights in the flat roof section of the extension — these make an enormous difference

• Bi-fold or sliding doors across the full width of the back wall

• A glazed roof lantern if the extension is deep

• Internal glazed partitions rather than solid walls to let light flow through

6. Ignoring Building Regulations

Building regulations aren't optional extras. They cover structural safety, fire protection, insulation, drainage, ventilation, and electrical safety. Every extension needs building regulations approval, and the work must be inspected at key stages.

Some homeowners skip building regs to save time or money. This creates a serious problem when you sell: your buyer's solicitor will flag the lack of a completion certificate, and you'll either need to apply for retrospective approval (which might not be granted if the work doesn't comply) or accept a reduced sale price. Regularisation applications cost more than doing it properly in the first place.

7. Poor Communication with Neighbours

Your extension will affect your neighbours — through noise during the build, potentially reduced light or privacy, and the general disruption of a building site next door. Homeowners who don't talk to their neighbours before submitting planning often face objections that delay or even scupper their application.

A simple conversation before you apply — showing them the plans, explaining the timeline, and being open about any potential impact — goes a long way. Most neighbours are reasonable when they feel respected. Springing a surprise on them is a recipe for conflict, and conflict during a build is stressful for everyone.

If party wall agreements are needed, serve the notices early. The party wall process takes time, and a dissenting neighbour can add weeks to your schedule.

The Common Thread

Most of these mistakes come down to rushing into the build without enough preparation. The planning and design stage feels slow and expensive, but every pound and every week you spend getting it right saves you multiples during the build.

Got a project you want to do right first time? We're always happy to have a no-obligation chat about your plans — even if you're months away from starting.