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5 min read Webhound Engine August 2, 2026
5 Mistakes Builders Make Quoting Extensions (And How to Protect Your Margins)
When you quote an extension, you aren't just pricing materials and labour. You are pricing risk.
Yet, most builders write quotes as if every job will go perfectly. No bad weather. No delayed materials. No clients changing their minds about the kitchen tiles halfway through the build.
When you quote for perfection, you pay for reality.
Here are the 5 biggest mistakes builders make when quoting extensions, and how to fix them to protect your profit margins.
This is the "Faff Factor". If the client is going to be living in the property during the build, add 15% to your labour cost. Period.
If you do not have a strict variation order policy written into your initial quote, you will do thousands of pounds of free work. Your quote must state: *Any work outside this scope requires a signed Variation Order before work begins.*
Instead of vague sums, specify exactly what the provisional sum covers (e.g., "Supply of kitchen units only, up to the value of £10,000. Fitting is included. Worktops, appliances, and sink are excluded and to be supplied by client").
Your payment schedule should keep you cash-flow positive at all times.
Stop writing quotes from scratch every night. Let Webhound Engine do the heavy lifting for you.
Yet, most builders write quotes as if every job will go perfectly. No bad weather. No delayed materials. No clients changing their minds about the kitchen tiles halfway through the build.
When you quote for perfection, you pay for reality.
Here are the 5 biggest mistakes builders make when quoting extensions, and how to fix them to protect your profit margins.
1. Failing to Price the "Faff Factor"
You priced the brickwork. You priced the roof. You priced the first fix. But did you price the hour you'll spend arguing with the client about where the plug sockets go? Did you price the extra time it takes to protect their existing hallway carpet every single morning?This is the "Faff Factor". If the client is going to be living in the property during the build, add 15% to your labour cost. Period.
2. Not Including a Strict Variation Order Policy
"While you're here, could you just..." Those six words destroy margins. A client asks you to move a radiator. It only takes two hours, so you don't bother quoting for it.If you do not have a strict variation order policy written into your initial quote, you will do thousands of pounds of free work. Your quote must state: *Any work outside this scope requires a signed Variation Order before work begins.*
3. Vague Provisional Sums
Putting "Kitchen: £10,000" is a recipe for disaster. The client goes to a high-end showroom, picks out £18,000 worth of quartz worktops and appliances, and expects you to cover the difference or get angry when you charge them more.Instead of vague sums, specify exactly what the provisional sum covers (e.g., "Supply of kitchen units only, up to the value of £10,000. Fitting is included. Worktops, appliances, and sink are excluded and to be supplied by client").
4. Back-loading the Payment Schedule
Many builders take a small deposit, fund the materials themselves, and leave a huge final payment at the end. If the client disputes the final payment, you've essentially paid to build their house.Your payment schedule should keep you cash-flow positive at all times.
5. Not Pricing for Waste and Skips
Waste disposal costs are skyrocketing. If you under-calculate the number of skips needed for a major dig-out, your profit goes straight into the landfill. Always add a 20% contingency to your waste disposal estimates.Stop writing quotes from scratch every night. Let Webhound Engine do the heavy lifting for you.