# Snagging and Retention: What to Hold Back at the End of an Extension

> How to snag a London extension properly, what a retention is for, and how to get the list closed without a last-week argument.

Author: Ben Murrell, Homeowners Project Manager · Published: 2026-10-09 · Category: Project Management
Canonical: https://www.homeownersprojectmanager.co.uk/blog/snagging-and-retention-extension

The end of a job is when you want the house back. It is also when the last 5% of the work can sit unfinished for months, because the builder's next job has already started and you have already paid. Snagging and retention are how you stop that being a favour you are asking for.

I am Ben Murrell. Closing the list is part of the job on every project I run, including the ones I join late.

## Snagging is a list, not a mood

A snag is a defect against what was agreed. A scratch in the glass. A door that does not latch. Paint on the ironmongery. A downpipe that misses the gulley. Silicon that has already failed. It is not a change of mind about the tile. Changes of mind at the end are variations, and they should be priced as variations. Mixing the two is how the list becomes an argument.

Walk it once, slowly, with the lights on, and write it down. Room by room. Outside as well: flashings, air bricks, paving levels, the junction where the new roof meets the old wall. Photograph anything you will want to point at later. Send one list, not a week of separate texts.

Then agree who does what, and by when. "We'll swing by" is not a date. A short list with dates is. If items need a specialist — the glazing company, the kitchen fitter — say so on the list, so they are booked rather than hoped for.

## What retention is for

Retention is a small share of the price, held back at completion and released when the list is done. On homeowner jobs it is often in the region of 2.5% to 5%, sometimes held for a defects period of a few months. The [JCT homeowner contract](https://www.homeownersprojectmanager.co.uk/blog/jct-contracts-explained-for-homeowners) is where this is normally written down. If your quote never mentioned it, you can still ask for it before you sign. After you have paid the last invoice, you are asking for a favour.

Retention is not a punishment and it is not a discount. It is the cost of the builder coming back. Builders who intend to come back do not fear it. The number should be enough to get someone else to finish the list if you have to, and not so large that you have underpaid the job.

Release it when the list is closed, not when you are tired of the conversation. If one item is genuinely outstanding — a part on order — release the rest and keep a sum that matches that item. Do not keep the whole retention hostage to a hinge, and do not give up the hinge to feel finished.

## The last week is where pressure peaks

You have been in a building site for months. The builder wants the final payment. You want the dust gone. That is when people sign a list off to get the keys back, then live with the scratch. It is also when new defects get called snagging so they do not have to be priced.

Someone on your side walks it with you, splits defects from extras, and holds the retention against the list you both signed. You still decide. You decide once, on paper, instead of renegotiating every Saturday. Catherine and Rob in Wandsworth talked about the job staying calm. The end of a job is where calm is most useful, because everyone is tired.

This sits inside the fee, which is 10–12% of the ex-VAT project cost depending on how hands-on you want the appointment. It is not a separate snagging invoice. If you are already at the end of a job and the list is drifting, say so when you get in touch. Joining late is messier than starting at the quote, and I will tell you if the leverage has already gone.

[Talk it through](https://www.homeownersprojectmanager.co.uk/contact) · 07525 474 849 · benmurrell83@gmail.com

## Keep reading

- [Payment schedule for a London extension](https://www.homeownersprojectmanager.co.uk/blog/payment-schedule-london-extension)
- [What a client-side project manager actually does](https://www.homeownersprojectmanager.co.uk/blog/what-does-a-client-side-project-manager-do)
- [Rear extension case study](https://www.homeownersprojectmanager.co.uk/projects/rear-extension-south-west-london)

---

Homeowners Project Manager — independent client-side project management for homeowners in South West London. Contact: 07525 474 849 · https://www.homeownersprojectmanager.co.uk/contact