How Long Does a House Extension Take? A Realistic 2026 Timeline

A house extension project has two distinct phases: the pre-construction phase (design, planning, and procurement) and the construction phase (the actual building work). The pre-construction phase is often longer than homeowners expect — and it's the phase over which the homeowner has the most influence, because delays in making decisions, providing information, or approving drawings directly extend the programme.

The construction phase is determined by the size and complexity of the extension. According to data from the Federation of Master Builders (FMB), the average construction time for a single-storey rear extension in the UK is 8–12 weeks. A double-storey extension takes 12–16 weeks. A wrap-around extension (rear and side) takes 14–20 weeks. These figures assume a competent contractor working continuously — they don't account for weather delays, material supply issues, or unforeseen ground conditions.

The planning route also affects the overall programme. An extension built under Permitted Development rights can start construction as soon as Building Regulations approval is granted (typically 4–6 weeks after submission). An extension requiring a householder planning application adds 8–13 weeks to the programme (the statutory determination period). An extension requiring the Neighbour Consultation Scheme (Prior Approval) adds 6–8 weeks.

The pre-construction phase covers everything from the initial design brief to the contractor starting on site. For a typical single-storey rear extension in Hertfordshire built under Permitted Development rights, the pre-construction phase takes 8–15 weeks. For an extension requiring a householder planning application, it takes 16–26 weeks. These timescales assume that decisions are made promptly and that no revisions are required.

The design phase involves a measured survey of the existing property, the preparation of existing and proposed drawings, and the development of the design to a level of detail sufficient for planning and Building Regulations submissions. For a straightforward single-storey extension, this typically takes 2–4 weeks. For a more complex project — a double-storey extension with structural alterations, for example — it may take 4–8 weeks.

The most common cause of delay in the pre-construction phase is the homeowner changing their mind about the design after drawings have been prepared. Every revision to the design requires the drawings to be updated, which takes time and may require re-submission to the LPA or Building Control. Agreeing the design brief clearly at the outset — and sticking to it — is the single most effective way to keep the programme on track.

Contractor procurement — obtaining quotes, comparing them, and signing a contract — typically takes 2–4 weeks. Rushing this process increases the risk of appointing a contractor who is not the right fit for the project. Taking too long increases the risk of the contractor's availability changing. A JCT Minor Works contract is the standard form of contract for residential extensions — it protects both parties and sets out clear obligations on programme, payment, and quality.

The construction phase of a single-storey rear extension typically follows this sequence: strip foundation excavation and concrete pour (1–2 weeks); blockwork walls to damp-proof course level (1 week); floor slab or beam-and-block floor (1–2 weeks); external walls to wall plate level (2–3 weeks); roof structure and covering (1–2 weeks); windows and external doors (1 week); first fix (electrical, plumbing, carpentry) (1–2 weeks); insulation and plasterboard (1 week); plastering (1 week); second fix (electrical, plumbing, carpentry) (1 week); tiling, flooring, and decoration (1–2 weeks). Total: 12–18 weeks for a typical 20–30 m² extension.

The construction programme can be compressed by overlapping trades — for example, starting the first fix electrical work while the plastering is still in progress in other areas of the extension. An experienced contractor will programme the work to minimise idle time and maximise the efficiency of each trade. However, there are dependencies that cannot be overlapped — the roof must be watertight before the first fix can start, and the plaster must be dry before the second fix can start.

Weather is the most unpredictable variable in the construction programme. Concrete cannot be poured in freezing conditions (below 2°C) without frost protection measures. Brickwork and blockwork cannot be laid in heavy rain. Roofing work is dangerous in high winds. In Hertfordshire, the winter months (November to February) are the most likely to cause weather-related delays — a programme that starts in October may experience 2–4 weeks of weather delays before reaching the roof stage.

The most effective way to keep an extension on programme is to make all design decisions before work starts. Every decision that is deferred to the construction phase — the kitchen layout, the bathroom fittings, the floor finish — creates a risk of delay. A contractor who is waiting for a decision about the kitchen layout cannot complete the first fix electrical work, which delays the plastering, which delays the second fix, which delays the completion.

A pre-construction meeting with the contractor — typically held 2–4 weeks before work starts — is the opportunity to agree the programme, confirm all design decisions, agree the payment schedule, and establish the communication protocol. This meeting is worth investing time in. A well-prepared pre-construction meeting reduces the risk of delays and disputes during the construction phase.

Regular site meetings during the construction phase — typically weekly or fortnightly — allow the homeowner and contractor to review progress against the programme, identify any emerging issues, and make decisions before they become delays. A contractor who is not willing to hold regular site meetings is a contractor who is not committed to keeping the programme on track.

The most common causes of delay in residential extension projects, based on FMB member surveys, are: unforeseen ground conditions (unexpected foundations, buried services, or contaminated ground — 35% of projects); material supply delays (particularly for bespoke items such as windows, doors, and structural steelwork — 28% of projects); design changes during construction (20% of projects); subcontractor availability (15% of projects); and weather (12% of projects).

Unforeseen ground conditions are the most disruptive cause of delay because they affect the foundations — the first stage of construction — and can require a redesign of the foundation system. A pre-construction ground investigation (a trial pit or borehole) can identify ground conditions before work starts and allow the foundation design to be adjusted accordingly. For extensions in areas with trees (which can cause soil shrinkage and heave), a ground investigation is strongly recommended.

Material supply delays have become more common since 2021, when global supply chain disruptions affected the availability of structural steel, timber, and insulation products. Lead times for bespoke windows and doors can be 8–12 weeks from order to delivery. Ordering long-lead items at the start of the project — rather than waiting until they're needed on site — is the most effective way to avoid supply-related delays.

TCM Building & Maintenance produces a detailed programme for every extension project before work starts — broken down by stage, with key milestones and decision points clearly identified. Our project managers hold weekly site meetings with the homeowner and update the programme after each meeting, so the homeowner always knows where the project stands and what decisions are needed.

For a client in Bushey who wanted a double-storey side and rear extension, our programme showed a total project duration of 32 weeks from design start to completion — 8 weeks for design and planning, 6 weeks for Building Regulations approval, 4 weeks for contractor procurement, and 14 weeks for construction. The project completed in 31 weeks — one week ahead of programme — despite a 3-day weather delay during the foundation stage. The client moved into the completed extension before Christmas, as planned.

Read our House Extension Cost Guide for detailed pricing information, or our Building Regs vs Planning Permission guide for information on the approvals process.