What is snagging on a building project?

Snagging is the systematic inspection of completed work to identify defects that should be fixed before the project is considered finished. On an extension it is a room-by-room, trade-by-trade check of every surface, fitting, socket, and structural element.

When should you snag your extension?

Snag after your builder considers the work substantially complete but before you release the final payment. Once the money has cleared, your ability to compel fixes drops sharply, so the timing is what gives the snagging list its weight.

Are cracks in a new extension normal?

Hairline and fine cracks at wall, floor, and ceiling junctions are normal and come from shrinkage and settlement as new and old structures move at different rates. Wider cracks, tapered cracks, or cracks visible on both sides of a wall should be assessed by a structural engineer.

What are NHBC tolerances?

NHBC Standards Chapter 9.1 sets out the accepted tolerances for UK residential construction finishes, such as how far a wall can be out of plumb or how flat plastering and tiling must be. They turn a subjective complaint into an objective standard you can measure against.

Can you withhold final payment for snagging defects?

Yes. You are entitled to withhold payment for unresolved defects, and a documented snagging list is the evidence that justifies it. A common approach is to hold back a retention covering snagging and the defects liability period rather than paying in full.

What is a defects liability period?

It is a period after practical completion during which the builder remains responsible for fixing defects that emerge as the building dries out and settles. A retention percentage is held back over this period and released once any new defects have been remedied.

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Phase 8 · Completion · Task 02 of 05

Extension Snagging Checklist: How to Inspect Your Builder's Work

A systematic room-by-room snagging guide for completed extensions, covering structural, functional, and cosmetic defects, NHBC tolerances, crack assessment, and how to hold final payment to enforce completion.

14 min readUpdated 2026Free with email

Infographic showing three categories of snagging defects: structural (red, must fix), functional (amber, must fix), cosmetic (green, negotiate), arranged as a severity scale with example items in each category

Prerequisites

Complete these first

The final payment to your builder is the most valuable thing you have. Once it's gone, your ability to get defects fixed goes with it. Builders who call back promptly when there's money outstanding have a habit of becoming hard to reach the week after final payment clears.

Snagging is how you protect that position. It's the systematic inspection of your completed extension before you sign off financially: a room-by-room, trade-by-trade assessment of every surface, fitting, socket, and structural element. Done properly, it tells you exactly what needs fixing, gives you a documented basis for withholding payment, and gives your builder a clear list of what they owe you.

Done badly (or done after you've paid), it's just a list of things that annoy you with no mechanism to get them fixed.

What this guide covers

01What Snagging Actually Means
02When to Do It, and Why Timing Matters
03Your Inspection Kit
04The Three Categories of Snags
05Structural Snags: What to Look For
06Cracks: When to Worry and When Not To
07Functional Snags: The Full Check
08Cosmetic Snags: Room by Room
09External Snags
10Presenting the List to Your Builder
11When the Builder Won't Respond
12Retaining a Percentage for the Defects Liability Period

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