Extension Project Management: A London Homeowner's Guide
Project managing a home extension means owning the whole job: design, approvals, budget, trades, sequence, safety and sign-off. You can do it yourself, hire an independent professional, or have your builder run it in-house. The right choice depends on your time, your appetite for risk and how complex the build is. This guide explains what the role involves, where London extensions most often go wrong, and how to decide who should take it on.
TL;DR
Project management is not an optional extra. RICS describes a home extension as a complex, major project, and somebody has to manage the design, budget, contract and statutory approvals.
Planning permission, building regulations approval and Party Wall notices are three separate processes. Each has its own timeline, and each can stall a start date.
A fixed price only protects you if the scope is defined and there is a written procedure for changes before they happen on site.
Get safety responsibilities in writing. Under CDM 2015, domestic client duties usually pass to your contractor unless you appoint a designer to manage the project.
A single accountable team, with a director overseeing the job, removes most of the hand-offs where delays and disputes begin.
Table of Contents
What extension project management actually covers
People often picture a project manager as someone with a clipboard checking that bricklayers turn up. In reality the work starts long before the first trade arrives, and a good share of it is paperwork and decision-making.
A typical extension has five phases, and the manager’s job changes in each.
Brief and feasibility. What do you need the space to do, what will the site allow, and what is your real budget including fees, VAT, fixtures and a contingency?
Design and approvals. Drawings, structural calculations, planning route, building regulations submission and any Party Wall notices.
Procurement. Tender documents, quotes compared like for like, and a written contract.
Construction. Programme, ordering, trade sequencing, inspections, variations and payments.
Completion. Snagging, certificates, guarantees and handover.
RICS sets out the same core duties: producing initial designs, cost consultancy to monitor the budget, and managing the contract so the project does not overspend. It also recommends professional oversight for navigating planning and building regulations.
The skills matter as much as the list. APM’s Adam Russell argues that the best managers keep several threads moving at once: closing off each step properly, acting decisively when things are uncertain, and managing specialists well. On a house extension, that means the structural engineer, the groundworker, the electrician and the plasterer all need the right information at the right moment.
Three ways to manage your extension
There is no single correct model. Here is how the main options compare.
Route | Who runs the project | Where it works well | Where it can struggle |
Do it yourself | You | Simple, single-storey work, with time to spare and some building knowledge | Time, missed details and liability that sits with you |
Independent professional (architect, surveyor or project manager) | A third party, separate from the builder | Complex or high-value projects, or when you want a check on the contractor | An extra fee and an extra party to coordinate |
Contractor-led, in-house | One firm covers design coordination, trades and oversight | Homeowners who want one point of accountability | Only as good as the firm’s systems and its transparency |
Managing it yourself
Self-managing can save money, but it is a serious commitment. Homeowners’ own forums are full of people asking how to project manage a single-storey extension with no building experience. The most common advice is to study the drawings until you understand what has to be built, including foundations, lintels and insulation specifications. If you cannot read those details, you cannot spot when something is built wrongly.
Be realistic about what you are taking on. You become the person who sequences trades, chases deliveries, makes urgent decisions on site and carries the consequences when a step is missed.
Appointing an independent professional
RICS advises appointing one of its members to act as project manager. They have the technical competence to deal with statutory authorities, manage contracts and advise on dispute resolution if a project hits serious delays. This route suits you if you want an independent voice checking the builder’s work and valuations.
Contractor-led management
Many London homeowners prefer one accountable firm that designs the build sequence, employs or directly controls the trades and manages the programme. It removes the gap between “the person who promises” and “the person who delivers”. It also puts a premium on how the firm works, which is why the questions later in this guide matter.
The approvals a project manager must line up
A surprising number of extension delays are administrative. They are also avoidable, because the rules are known in advance.
Planning permission
Do not assume yours will be granted because your neighbours have built something similar. RICS warns that regulations change, so similar extensions next door are no guarantee. For rear extensions, our guide to rear extension planning in London, including the 3m limit and 6m consultation explains how the distances work in practice.
Building regulations
Building regulations approval is a separate matter from planning. As Haringey’s building control guidance explains, it is needed for most structural work, including extensions and removing load-bearing walls. The completion certificate matters later: you will want it for selling, insuring and mortgaging the property.
If you want more detail, we have written about building regulations for West London homeowners, building control sign-off and the drainage approvals needed before an extension.
Party Wall notices
Many homeowners think the Party Wall etc. Act 1996 only applies to a shared wall. The government’s explanatory booklet shows it can also cover excavation for new foundations near a neighbouring structure, with distances of 3 or 6 metres depending on depth and angle. Notice normally has to be served one to two months before work starts, depending on the type of work, so it belongs on the programme from day one.
The same booklet suggests discussing your plans with adjoining owners before notice is served, ideally with your professional adviser. In dense West London streets, that early conversation is often what keeps a start date intact. Note that the Act is a civil matter between owners, so the council will not normally step in to settle a disagreement.
Health and safety under CDM 2015
This is the one many homeowners miss. Under the Construction (Design and Management) Regulations 2015, domestic clients have legal duties. The HSE explains that these duties normally pass to the contractor or principal contractor, unless you have a written agreement with a designer, such as an architect, to manage the project. So do not assume your builder has taken it on. Ask them to confirm in writing who holds the role.
Where the money goes wrong
Budget control is the heart of project management, and it is where homeowners most want reassurance. The Planning Portal’s advice on financing your extension is to prepare a budget and calculate costs carefully so you are not faced with spiralling bills.
A fixed price is only as good as its scope
Three quotes are standard advice, and it is good advice. But a fixed price usually covers a defined scope. What really protects your budget is knowing what happens when the scope changes. Citizens Advice’s pre-work checklist walks through checking permissions, finding good traders and comparing quotes. Add these questions of your own:
How are variations priced, and who signs them off before work begins?
What is included for provisional sums such as flooring, tiling and sanitaryware?
How is the contingency held and reported?
What does the payment schedule look like against completed stages?
It is also worth reading about how cost overruns begin in planning, because most surprises trace back to decisions made, or avoided, before site work begins. Unresolved design details, vague specifications and undecided finishes all become variations later.
Put it all in writing
In August 2025, Citizens Advice reported more than 700 complaints a week about home improvements, with over half of the 19,281 annual complaints relating to poor quality service. Its advice is to insist on a written contract covering timings, cleaning up and how subcontractors are handled.
If a dispute does arise on a project built under a standard JCT contract, RICS Homeowner Adjudication offers an alternative to court, typically resolving matters within 21 days. A recognised contract form makes that kind of route available to you.
A practical programme for a London extension
Here is a sequence you can use to check any project plan, whoever manages it.
Confirm the brief and budget, including fees, VAT and contingency.
Commission measured surveys and drawings, then have the structural design coordinated with them.
Choose the planning route and submit, or confirm that your project fits permitted development.
Serve Party Wall notices early, once you have talked to neighbours.
Submit building regulations and agree who deals with drainage approvals.
Tender on a defined scope, with provisional sums kept as few as possible.
Sign a written contract, with a variations procedure and a payment schedule linked to stages.
Confirm CDM roles in writing.
Hold weekly progress meetings during construction and keep a written record of decisions.
Snag, certify and hand over, collecting the completion certificate and all guarantees.
If any step on this list is unowned, that is where your project is most likely to slip.
Our view: why oversight beats optimism
We have been refurbishing and extending homes across West and Central London since 2006. In that time the pattern has been clear: projects rarely go wrong because of one dramatic failure. They drift because small decisions go unrecorded, trades arrive out of sequence or nobody has the authority to say “stop, let’s resolve this first”.
That is why we run projects in-house, with a director overseeing each one, rather than passing work along a chain of subcontractors. When the person responsible for your project is also the person who answers your questions, decisions are quicker and accountability is simple. We also price from an itemised quotation after a site survey, so you can see what you are paying for before you sign.
Our advice to any homeowner, whoever you hire: ask who will be on site, who signs off changes, and who you call when something is not right. If the answers are clear and written down, you are in good hands. If they are vague, keep looking.
— Mateja
Questions to ask before you appoint anyone
Who is my single point of contact, and how often will we meet on site?
Which trades are employed directly, and which are subcontracted?
How are variations quoted and approved?
Who holds the CDM duties, and will you confirm this in writing?
Which insurance and warranty cover applies, and can I see the documents?
Who handles building control inspections and the completion certificate?
A contractor who welcomes these questions is usually one who has good answers.
Planning an extension in West or Central London?
If you are weighing up an extension in Fulham, Chelsea, Kensington, Chiswick, Hammersmith or Notting Hill, we can help you shape the plan before a single trade is booked. Our extension service covers rear, side and wrap-around extensions, with in-house delivery and director oversight. We offer a rapid site visit, then an itemised quotation. If you are also considering the space above, take a look at our loft conversion service, or get in touch through our homepage to arrange a visit.
FAQ
Do I need a project manager for a house extension?
Not legally, but somebody must do the job. RICS describes extensions as complex projects, so you can either manage it yourself, appoint an independent professional, or choose a contractor who manages the work in-house. The more complex the build, the stronger the case for dedicated oversight.
Who is responsible for health and safety on a domestic extension?
Domestic clients have duties under CDM 2015, but the HSE says these normally pass to your contractor or principal contractor. They stay with you if you appoint a designer to manage the project only by written agreement. Ask your builder to confirm the arrangement in writing.
Is planning permission the same as building regulations approval?
No. They are separate processes, and most extensions need building regulations approval even where planning permission is not required. You should receive a completion certificate at the end, as you will need it when you sell, insure or remortgage.
How do I stop an extension going over budget?
Define the scope in detail before you tender, keep provisional sums low and agree in writing how variations are priced and approved. Hold a contingency and review costs at every progress meeting. Most overruns come from changes decided late.
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