Main Contractor vs Project Manager in London 2025: Which Delivery Route is Right for Your Project?

How the main contractor route works: the homeowner (or the architect on the homeowner's behalf) appoints a single main contractor to carry out the entire building project. The main contractor: agrees to complete the works for an agreed contract sum (fixed price, or a schedule of rates for cost-reimbursable contracts); takes responsibility for: engaging all subcontractors (groundworkers, structural steel erectors, brickwork gangs, roofers, electricians, plumbers, joiners, plasterers, decorators) and paying them; procuring and delivering all materials to site; sequencing and programming the works so that each trade follows the previous one efficiently; managing health and safety on the site (as the Principal Contractor under CDM 2015); resolving problems, delays, and quality issues during construction; achieving practical completion by the agreed date. The homeowner deals with the main contractor for all day-to-day construction matters — they do not need to coordinate individual trades or manage the subcontractors directly. If there is an architect administering the contract (Stage 5), the architect also interfaces with the main contractor — certifying payments, issuing instructions, and monitoring quality and programme. Advantages of the main contractor route for London domestic projects: single point of contractual responsibility: if anything goes wrong (a subcontractor fails to complete their work, materials are delayed, the programme slips), the main contractor is responsible — not the homeowner. The homeowner has a single contract, a single counterparty, and a single party to hold to account for cost, quality, and time. Coordination and sequencing managed by the contractor: coordinating 8-12 different trades in the correct sequence, with the correct overlaps, managing the programme to avoid expensive delays — this is the main contractor's core competency. A homeowner attempting to do this themselves without construction experience is very likely to make costly errors (a plasterer arrives before the electrician has finished first-fix; a flooring contractor starts before the wet room waterproofing is tested). Fixed price certainty: a fixed price main contractor appointment provides the homeowner with cost certainty (subject to variations — scope changes). This is important for budget management and financing. Principal Contractor under CDM 2015: if the project has more than one contractor or is notifiable (more than 30 working days with more than 20 workers simultaneously, or more than 500 person-days), a Principal Contractor must be appointed under CDM 2015 (the Construction Design and Management Regulations 2015). The main contractor typically takes this role — relieving the homeowner of the statutory CDM duties that would otherwise fall to them as 'client'. Disadvantages of the main contractor route: the main contractor's mark-up: the main contractor adds a mark-up (overhead and profit) on all subcontractors and materials they procure — typically 10-20% on subcontractor costs and 5-15% on materials. This mark-up is the price for the coordination, risk-taking, and programme management that the main contractor provides. A homeowner appointing subcontractors directly might save 10-20% on those subcontractor and material costs — but they take on the coordination risk themselves. Less transparency: in a fixed-price main contractor appointment, the homeowner typically does not see the subcontractor costs or margins — they see only the total contract sum and the payment schedule. Cost-plus or open-book arrangements (where the homeowner sees all the underlying costs and pays cost + an agreed margin) are available with some contractors but are less common for fixed-price domestic projects. Quality of subcontractors: the main contractor selects the subcontractors — the homeowner does not typically get to choose or approve individual subcontractors (unless the contract specifically reserves that right). An experienced main contractor with a trusted subcontractor base provides consistent quality; a less experienced or less well-networked contractor may use lower-quality subcontractors to protect their margin.

How the project manager delivery route works: instead of appointing a main contractor, the homeowner appoints individual subcontractors (groundworkers, roofers, bricklayers, first-fix M&E, etc.) directly — and uses a project manager (PM) to coordinate, programme, and supervise the trades on the homeowner's behalf. The project manager's role typically includes: preparing a programme and sequencing plan; obtaining quotes from individual trades and subcontractors on the homeowner's behalf; recommending subcontractor appointment; coordinating site access, deliveries, and sequencing; monitoring quality (visiting site regularly and reporting to the homeowner); managing variations and the change control process; liaising with the architect or Building Control where relevant. The project manager does NOT: take contractual responsibility for the construction — the contracts with each subcontractor are between the homeowner and the subcontractor directly; manage health and safety as Principal Contractor (the homeowner may need to appoint a separate CDM Principal Contractor, or may be the Principal Contractor themselves — with all the associated statutory duties); guarantee cost or programme. Project manager fee structure: typically either: a percentage of construction cost — typically 8-15% of total construction cost; a fixed fee — negotiated based on project size and complexity. For a £150,000 London extension, a project management fee of 10% = £15,000. Advantages of the project manager route: potential cost saving: by removing the main contractor's 10-20% mark-up on subcontractors and materials, the homeowner can potentially save 8-15% of the total construction cost. However: this saving is not guaranteed and depends on the homeowner's and PM's ability to source and negotiate good subcontractor rates — a good main contractor with long-term subcontractor relationships and buying power may achieve similar or better rates than a homeowner appointing directly. Transparency of costs: the homeowner sees all subcontractor and supplier quotes and invoices directly — there is full cost transparency. Control over subcontractor selection: the homeowner (with the PM's recommendation) can choose and approve each subcontractor — useful if the homeowner has established relationships with specific trade contractors or has quality preferences that a main contractor might not share. Disadvantages of the project manager route: risk stays with the homeowner: if a subcontractor fails to complete their work (insolvency, abandonment, poor quality), the homeowner is the party at risk — they have the direct contract with the subcontractor and must deal directly with the consequences. Coordination risk: the PM coordinates but does not guarantee the programme. If trades overrun, the PM cannot financially penalise the subcontractors for the knock-on costs — only the homeowner (via their direct contract with each subcontractor) can seek redress, and this requires the homeowner to have adequate contractual protections in place with each individual trade contractor. CDM Principal Contractor: in a multi-trade project where the PM does not take the Principal Contractor role, the homeowner may be the de facto Principal Contractor (or may need to appoint a separate PC) — with all the associated CDM statutory duties (construction phase plan, F10 notification, welfare facilities, site management obligations). PM availability and continuity: the PM is not on site every day — they are typically visiting for inspections and coordination (one or two days per week for a large project). The homeowner must be available to make decisions when needed, and the PM must have the right contact details and a clear instruction channel with each subcontractor. When is the project manager route appropriate in London? The PM route is generally more appropriate when: the project is very large (above approximately £500,000-£1 million in value) and the main contractor's 10-20% mark-up represents a significant saving; the homeowner is an experienced property developer or investor with existing subcontractor relationships and understands the coordination risks; the project has a very clear and fully detailed specification and programme, reducing the coordination complexity for the PM; the homeowner has the time and appetite to be more closely involved in the daily decision-making of the project. For most London homeowners building a £50,000-£300,000 extension or refurbishment for the first time, the main contractor route is strongly recommended — the coordination certainty and single point of responsibility are worth the 10-20% margin the main contractor charges.

The design-and-build (D&B) principal contractor model: the third delivery route — and the model used by RCB Design & Build for most London projects — combines the benefits of the main contractor route with integrated design and redesign capability, pre-construction coordination, and a single contractual point of responsibility from the client's first conversation through to practical completion. How a London design-and-build principal contractor works: the D&B contractor takes responsibility for: the design (or redesign where needed) — developing the project from the client's brief through concept, planning, and technical design; the pre-construction coordination — procuring subcontractors, obtaining quotes, programming the works, coordinating CDM compliance, party wall procedures, scaffolding licences; the construction delivery — managing the site, all subcontractors, all materials, and the programme; practical completion and defects liability. In a D&B appointment, the employer's requirements (what the client wants to achieve — the brief) and the contractor's proposals (the D&B contractor's design and construction response to the brief) are the key documents. The client has a single point of responsibility for everything — design, planning, construction, and handover. Benefits of the D&B principal contractor route for London homeowners: integrated design and cost: the design and the cost plan are developed together — the D&B contractor can flag where design decisions increase cost (and offer alternatives) before the design is finalised. Redesign capability: where the client's initial brief involves a layout or specification that would be expensive or impractical to build, the D&B contractor can redesign — proposing an alternative approach that achieves the same functional goal at lower cost or greater buildability. Single point of responsibility: one contract, one counterparty, one party to hold to account — from the initial design meeting to the defects liability period. Planning experience: a D&B principal contractor with strong London planning experience can manage the planning application process as part of the overall service — coordinating the architect's drawings, the planning portal submission, pre-application advice, and planning condition discharge. Pre-construction clarity: the D&B contractor coordinates all pre-construction activities (structural engineer, party wall surveyor, scaffolding licence, section 169 highway notice) before construction starts — reducing the risk of site delays due to missing approvals. Principal Contractor role under CDM 2015: the D&B contractor takes the Principal Contractor role, relieving the homeowner of the associated CDM statutory duties. Choosing a D&B principal contractor in London: key questions to ask a D&B principal contractor: Do you provide a fixed-price proposal or a cost-plus arrangement? (Fixed price is preferred for most homeowners.) What does the design stage of the service include — are architectural drawings, structural engineer coordination, and planning application management all included or separately charged? What is your experience with similar projects in the relevant London borough? Can you provide 3 references from clients whose projects were similar to mine — including telephone references I can call? Are you FMB, TrustMark, or Checkatrade registered? Do you hold PLI of at least £2 million and ELI? What is your standard payment schedule and retention arrangement?