Architect
Works for You (the homeowner), unless you have appointed a design-and-build contractor who employs them.
What they do
- Designs the project and produces drawings
- Handles planning applications and often the building-regulations submission
- May offer contract administration during the build — priced and scoped separately
When to appoint
Early — ideally before you have committed to a builder. On a typical SW London terrace extension or loft, appoint once the brief is clear and before party wall notices need serving off settled drawings.
What they are not
Not usually scrutinising builder pricing line by line, chasing day-to-day site decisions, or protecting your budget against creep once work starts — unless you have specifically appointed them for contract admin.
Structural engineer
Works for You (or sometimes appointed via the architect). Their duty is the structure, not the builder’s programme.
What they do
- Calculations and structural drawings for steels, openings, foundations and roofs
- Designs temporary works where needed
- Produces information building control and the steel fabricator need
When to appoint
Once the architectural layout is settling — early enough that steel lead times do not sit on the critical path. For most rear extensions and loft conversions on Victorian terraces, you will need one.
What they are not
Not a project manager, and not responsible for comparing builder quotes or running the site.
Party wall surveyor
Works for Impartially under the Party Wall etc. Act 1996 — either as an “agreed surveyor” for both sides, or as your surveyor alongside the neighbour’s.
What they do
- Advises on whether the Act applies
- Serves or responds to notices
- Prepares the Party Wall Award where there is a dispute or a dissent
When to appoint
As soon as the drawings are firm enough to describe the work. Notices need at least two months before work on a party wall (one month for excavation). Leaving it until the builder is booked is the most common way a start date slips.
What they are not
Not planning permission, and not building control. Skipping the Act can stop the build — see the full guide linked below.
Building control
Works for Public interest — either the local authority or a registered private approved inspector. They do not work for you or the builder.
What they do
- Checks the work meets building regulations
- Inspects at set stages
- Issues completion / final certificate when satisfied
When to appoint
Submit before relevant work starts. Inspections are booked to the programme — missed inspections mean opening work back up.
What they are not
Not a substitute for a clerk of works or client-side manager. They check compliance, not whether you are getting value or whether the quote matches what is being built.
Main contractor (builder)
Works for Their own business. A good one manages their trades, programme and margin well — that commercial interest is why you still want someone on your side of the table.
What they do
- Prices and builds the works
- Coordinates their own subcontractors
- Responsible for workmanship, health and safety on their site, and delivering what the contract says
When to appoint
After drawings and (ideally) structural information are good enough for a like-for-like tender — and after you have compared quotes properly, not just totals.
What they are not
Not appointed to minimise your spend. Variations, provisional sums and exclusions are where budgets move; those need reading from your side.
Client-side project manager
Works for You only. No commercial relationship with the builder.
What they do
- Reviews quotes and scope before you sign
- Helps get builders onto a like-for-like comparison
- Coordinates the team, holds the programme, and tracks variations
- Sources materials at trade prices where that helps
- Stays through snagging and handover
When to appoint
Most useful before a builder is appointed. Still valuable mid-project if you need someone on the ground — especially if you cannot be on site yourself.
What they are not
Not the architect, not the builder, and not building control. The point is the independent seat on your side of the table.