Construction Programme Explained: How Long Your Project Will Take

**Why 'how long will it take?' needs two answers**:

Every project has two distinct programme elements:

1. *The on-site programme*: The duration of actual construction works from site start to practical completion 2. *The pre-construction programme*: The activities that must happen before the contractor sets foot on site — design, planning, Building Control, party wall, and structural engineering

Homeowners who ask 'how long will my extension take?' are usually thinking of the on-site programme. Contractors who quote a 12-week programme are usually answering that question. But the full journey from 'I've decided to do this' to 'the extension is complete' is typically 6–14 months longer than the on-site programme alone.

**The pre-construction programme — London typical activities and durations**:

| Activity | Who does it | Typical duration | |---|---|---| | Appoint architect / architectural technician | Client | 1–2 weeks | | Concept design and drawings | Architect | 2–4 weeks | | Planning application preparation | Architect | 1–2 weeks | | Planning application determination | LPA | 8–14 weeks (statutory 8 weeks; in practice up to 14 for London LPAs) | | Structural engineer appointment | Client / Architect | 1 week | | Structural engineer calculations | SE | 2–4 weeks (after drawing set agreed) | | Building Regulations Full Plans application | Architect / SE / contractor | 1 week to submit | | Building Regulations plan checking | BC body | 5 weeks statutory; 3–4 weeks for Approved Inspector | | Party wall notices served | Client / party wall surveyor | Day 1 — serve as early as possible | | Party wall response period (Section 3) | Adjoining owner | 2 months minimum | | Party wall Award (if dissent) | Surveyors | +2–6 weeks after dissent | | Tender — prepare scope and send to 3 contractors | Client / Architect | 1–2 weeks | | Tender — contractors price and return | Contractors | 2–4 weeks | | Tender assessment and appointment | Client | 1 week | | Contractor mobilisation (notice period, SSIP, insurance, sub-contractor bookings) | Contractor | 4–8 weeks |

• *Practical reality*: Many of these activities can run in parallel — structural engineering during planning determination; Building Control application during tender period; party wall notices served on day 1 of design. A well-coordinated pre-construction programme can compress these activities to approximately:
• •Minimum: 4–5 months (where no planning is needed, neighbours consent immediately on party wall, and a contractor is available without a long mobilisation lead)
• •Typical: 6–9 months (planning application with standard 8–10 week determination + Building Control + party wall + contractor mobilisation)
• •Extended: 9–14 months+ (planning refusal and appeal, complex party wall dispute, multiple rounds of Building Control comments, or contractor with a long mobilisation queue)

**Serving party wall notices as early as possible**:

The party wall process is entirely separate from planning and Building Control — it runs in parallel. The Section 3 notice period is 2 months minimum. If both neighbours dissent and each appoints their own surveyor, the Award takes a further 2–6 weeks. Serving party wall notices on day 1 of the design process — rather than after planning permission is obtained — can save 2–4 months on the total programme.

**Realistic on-site programme durations for London residential projects (2025)**:

All durations assume: contractor has an experienced site team; materials are ordered with adequate lead times; no significant unforeseen conditions (buried services, poor ground, unexpected structural issues); and Building Control inspections are available without excessive wait times.

• **Single-storey rear extension (30–40m², open plan kitchen/dining)**:
• •Strip-out and demolition: 2–3 days
• •Excavation and foundation concrete: 1 week
• •Superstructure (blockwork or timber frame): 2–3 weeks
• •Flat or pitched roof structure and weatherproofing: 1–2 weeks
• •First-fix M&E (electrical, plumbing): 1.5 weeks
• •Windows and external doors: 1 week (after frame is set)
• •Insulation, first-fix carpentry: 1 week
• •Plastering and skim: 1.5 weeks (+ 1 week drying)
• •Second-fix M&E, carpentry, decorating: 2 weeks
• •Kitchen installation and finishing: 1–2 weeks
• *Total on-site: 14–18 weeks*

**Single-storey extension + ground floor refurbishment**: *Total on-site: 18–24 weeks*

**Single-storey rear + side return extension (wrap-around)**: *Total on-site: 18–24 weeks*

• **Double-storey rear extension**:
• •Significantly more complex due to structural requirements at first floor, scaffolding requirements, and coordination with upper floor rooms during construction
• *Total on-site: 20–28 weeks*

**Loft conversion — Velux**: *Total on-site: 8–12 weeks*

**Loft conversion — box dormer**: *Total on-site: 10–14 weeks*

**Loft conversion — L-shaped dormer**: *Total on-site: 14–18 weeks*

**Loft conversion — mansard**: *Total on-site: 16–22 weeks*

**Full flat refurbishment (2-bed flat, occupied during works)**: *Total on-site: 10–14 weeks*

**Full flat refurbishment (vacant, full strip-out and refit)**: *Total on-site: 14–18 weeks*

**Full Victorian terrace refurbishment (3-bed, strip-out to shell and refit)**: *Total on-site: 20–30 weeks*

**HMO conversion (3-bed to 5-bed, fire compliance, en-suites)**: *Total on-site: 12–18 weeks*

**Garage conversion (single, to habitable use)**: *Total on-site: 6–10 weeks*

**Basement conversion (shell condition to completion)**: *Total on-site: 16–28 weeks (depending on excavation and waterproofing complexity)*

**Critical path items that extend the on-site programme**:

• •*Structural steel fabrication and delivery*: Typically 4–8 weeks from order to delivery. If not ordered early enough, the structural opening cannot be formed and the programme stalls.
• •*Bespoke windows and doors*: 6–12 weeks lead time for aluminium or timber sliding or bifold doors. Order when the frame is measured — not when you want them.
• •*Kitchen lead time*: Bespoke fitted kitchens from trade suppliers: 6–12 weeks. IKEA: typically 2–4 weeks from order to delivery. Never assume kitchens are available off-the-shelf for delivery in less than 4 weeks.
• •*Building Control inspection availability*: During busy periods, BC inspectors may have 1–2 week wait times for inspection bookings. Build buffer into the programme at inspection-dependent stages.

**Setting a realistic programme at the outset**:

• The most effective way to manage construction programme expectations is to produce a written programme at the start of the project that shows:
• •All pre-construction activities (design, planning, BC, party wall, structural engineering, tender)
• •The planned site start date
• •On-site milestone dates (substructure, superstructure, weathered-in, first-fix complete, plastering, second-fix, practical completion)
• •Key material procurement milestones (steel order date, window order date, kitchen order date)
• •Building Control inspection stages

This programme should be prepared by the architect or contractor before the contract is signed and should form a contract document (the JCT Minor Works contract has a provision for a programme to be attached). A written programme creates accountability — both for the contractor (to hit milestones) and for the homeowner (to make decisions, supply materials, and respond to contractor queries promptly).

**The most common causes of programme slippage in London residential projects**:

1. *Late decision-making by the homeowner*: Tiles, kitchen units, sanitaryware, flooring, and ironmongery must be specified and ordered before the contractor needs them — often 6–10 weeks ahead. Late material decisions are the most common cause of practical delay in domestic refurbishments.

2. *Unforeseen site conditions*: Buried drainage routes, unexpected structural issues (deteriorated floor joists, incorrect load assumptions), asbestos discovery (common in pre-2000 London houses — artex ceilings, floor adhesives, pipe insulation), and underground obstructions (old Victorian wells, coal holes, collapsed drains) all require programme allowance.

3. *Structural steel delays*: Ordering structural steel too late is a very common programme delay. The contractor should order steel as soon as the structural engineer's scheme design is agreed — not when the structural calculations are complete.

4. *Party wall delays*: Uncooperative adjoining owners who appoint surveyors who then prolong the Award process can delay the start of notifiable works significantly. There is no contractual shortcut — the statutory process must complete.

5. *Planning delays*: LPA determination of a householder application taking 14+ weeks (common in under-resourced London LPAs) when the contractor's programme assumed 8 weeks.

**The 10% contingency rule for programme as well as cost**:

Just as 10–15% cost contingency should always be built into a project budget, 10–15% programme contingency should be built into the on-site programme. A 14-week extension programme should be planned to practical completion 15–16 weeks, not 14 weeks. This buffer absorbs the minor delays (inspection wait times, material delivery slippage, a wet week when external work is critical-path) that are inevitable on any real project.