Do I Need a Project Manager for My Extension?

You need a residential project manager when you will not be on site, when more than one builder is pricing, or when the contract is large enough that a variation log matters. You often do not, if the job is small, the builder is already trusted, and you have the time to read the quotes yourself. Where you do appoint one in London, a client-side fee here is 10–12% of the project cost, paid by you rather than folded into the builder’s price.

"Do I need a project manager for my extension?" is one of the first questions people ask me, usually with planning drawings on the kitchen table and three builder quotes that do not agree with each other.

The honest answer is: not always. There is no legal requirement to have one, plenty of single-storey extensions in London are built well without one, and I would rather tell you that now than sell you something you do not need. But there is a clear set of situations where an extension without anyone on your side tends to go wrong, and they are very common on South West London terraces.

I am Ben Murrell. I started as a carpenter, ran a residential building company, and now work only for homeowners as an independent, client-side project manager. Here is how I would decide, if it were my house.

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Someone is already managing your extension. The question is who they work for

Every extension has management in it somewhere. The useful question is not "do I need a project manager?" but "who is managing this, and whose interests do they represent?"

• Your builder manages their own site. A good main contractor runs their trades, orders materials and books the building control inspections. Any "project manager included" in a builder's or design-and-build package works for that company. They can be excellent at sequencing, but they are not paid to challenge their own employer's price or variations. I cover that difference in independent project manager vs design-and-build.
• Your architect may administer the contract. If you appoint them for the construction stage (RIBA Stage 5), they inspect at intervals, issue instructions and certify payments. That is valuable, but it is a different job from day-to-day management. See project manager vs architect contract administration.
• Or it is you. If nobody else has the job, you are the one comparing quotes, answering the builder's daily questions, checking whether a payment request matches the work done, and deciding what to do when the steel will not sit where the engineer drew it.

Health and safety law is one part you do not carry alone. Under the CDM Regulations 2015, the Health and Safety Executive says a domestic client's duties normally pass to the contractor (or the principal contractor where there is more than one), unless you have a written agreement for your designer to take them on. The commercial and practical management, though, sits with whoever you appoint, or with you by default.

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When you probably do not need a project manager

In these situations I would usually tell you to save the fee:

• A small, well-defined job. A simple single-storey rear extension with complete drawings, no party wall complications and a straightforward structural opening.
• A builder you already trust. Someone who has done good work for you or a neighbour in the same kind of house, and whose quote is itemised and clear.
• You have the time. You or your partner can take calls during the working day, be on site at key stages and read a quote line by line.
• Your architect is staying on. If they are appointed through construction, visit regularly and you are happy with how they run it, adding another layer can be duplication.

If that is you, put the money into contingency instead. You will still find the builder quote comparison checklist and how to check a builder before you sign useful.

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When you probably do

The case for independent help gets stronger with each of these that applies:

1. The build is heading towards six figures. Once provisional sums become real prices, many London extensions get there. The bigger the contract, the more a missed exclusion or an unpriced variation costs.
2. Your quotes are not comparable. If three builders' totals are far apart and you cannot say why, the gap is usually scope: what one includes and another leaves as a provisional sum or excludes altogether. That is exactly what I look at first. See provisional sums and PC sums explained.
3. The house is old and the site is tight. Victorian and Edwardian terraces in Wandsworth, Fulham, Clapham and Putney bring shallow footings, shared walls, drainage under the extension footprint and steelwork into original brickwork. Surprises at groundworks are where budgets are most often lost.
4. There is planning or party wall complexity. A conservation area, an Article 4 direction, or neighbours on both sides needing party wall notices all add steps where something can slip.
5. You are not available. Full-time jobs, travel, a new baby, or managing the job from another city. Builders need decisions on the day, not at the weekend.
6. Nobody is on your side commercially. On a design-and-build package, or when your architect's appointment ends at Building Regulations drawings, there is no one whose only job is to protect your budget once work starts.
7. You are living in the house. Living on site makes delays more painful and decisions more frequent. See living in your house during a renovation.

A quick self-test: if three or more of those apply, it is worth at least a conversation with an independent project manager before you sign a building contract. If only one applies, you can probably manage it with good paperwork and a good builder.

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What a project manager actually does on an extension

On my projects the work falls into three stages:

• Before anyone is on site. Reading the drawings against the quotes, getting builders pricing the same scope, questioning provisional sums, checking the builder (company, insurance, references) and getting a proper written contract in place, usually a JCT homeowner contract, with a payment schedule and a variation process.
• During the build. Regular site visits, sorting problems directly with the builder in their language, keeping a live budget, pricing and agreeing variations before they are built, checking payment requests against progress, and sourcing bigger-ticket materials at trade rather than retail prices where it helps.
• At the end. Snagging, building control sign-off, certificates and closing out the account with the builder.

There is a fuller version in what an extension project manager does week by week, and you can see a real example in my Wimbledon kitchen and side extension case study.

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What it costs, and whether it pays for itself

Market rates vary widely. Checkatrade's house renovation project manager cost guide (updated September 2026) puts project managers at 15–20% of total renovation spend, averaging 17.5%, or £100–£200 an hour for ad hoc help.

My own fee is 10–12%, depending on how involved you want me to be. The project manager cost page explains how that works, with a worked example, and the renovation fees calculator puts it alongside the architect, engineer, party wall and building control fees so you can see the whole picture.

If you are weighing it up, four practical guides help: how much time running your own extension actually takes, what a builder's extra should cost, a payment schedule for a London extension and VAT on a house extension.

Whether it pays for itself depends on the job. Sometimes it does and sometimes it does not: on a small, straightforward job with a builder you trust, the maths is thin, and I will say so. There is a line-by-line worked example in does a project manager fee pay for itself?. If you want to see where an extension budget typically drifts first, try the extension cost overrun calculator.

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A middle route: help only where it matters

You do not have to choose between full management and nothing. The two points where independent input does the most good are:

• Before you sign. A review of your drawings and quotes, so you know what is missing, what is a guess and what is a fair price.
• Contract set-up. Getting the contract, payment schedule and variation process right, so the build has rules before it has problems.

If you only want a second opinion, send me your builder's quote. The quote review is free, and if the quote is sound I will tell you that too.

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Talk it through

I work across South West, West and Central London, including Wandsworth, Fulham, Clapham, Putney, Battersea and Wimbledon. If you are weighing up whether your extension needs a project manager, send me a message or call 07525 474 849. There is no fee for the conversation, and if you do not need me I will say so.

Before you budget, the extension cost calculator gives a sourced London build range by type and specification (ex VAT).

General guidance, correct as of October 2026. Fees, regulations and market rates change. Check the current position and take advice on your own project.

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Common questions

Is a project manager a legal requirement for a house extension? No. Nothing in planning or Building Regulations requires one. Under CDM 2015, the HSE says a domestic client's health and safety duties normally pass to the contractor or principal contractor. A project manager is a choice about how much commercial and practical support you want on your side.

Can my builder project manage my extension? Your builder will manage their own site, and a good one does it well. What they cannot do is act independently for you on their own price, variations and payment requests. That is the gap a client-side project manager fills.

Can my architect project manage the build instead? Many architects offer contract administration through construction, which covers inspections, instructions and payment certificates at set intervals. It suits design-led and listed projects. It is not usually daily site management or supplier sourcing, so check exactly what their appointment includes.

At what budget does a project manager make sense? There is no fixed threshold. Complexity, how comparable your quotes are and how available you are matter more than a round number. As a rough guide, the case gets strong once a build is heading towards six figures, the house is a period terrace, or you cannot be the daytime decision-maker.

When should I bring a project manager in? Before you sign a building contract, ideally while quotes are coming in. Most of the value is in getting the scope, prices and contract right before work starts. Coming in mid-build is still possible, but some decisions will already be fixed.

Do you only work on extensions? No. I manage extensions, loft conversions and full renovations for homeowners in South West, West and Central London, always appointed by the homeowner and never by the builder.