How Long Does a House Extension Take? Realistic Timelines

The timeline most people do not expect

The question every homeowner asks first is "how long will the build take?" But the build is only part of the story. Before a single spade hits the ground, your project must pass through a series of pre-construction phases - design, approvals, engineering, procurement - that typically take as long as the construction itself. Failing to understand and plan for these phases is the single most common reason extension projects feel like they are taking forever.

At RCB Design & Build, we provide clients with a realistic end-to-end programme at the project review stage, covering every phase from initial design through to handover. As FMB and TrustMark accredited contractors, we have delivered hundreds of extensions across London and the South-East and our timelines are based on real project data, not optimistic guesses. This guide breaks down where the time actually goes.

Pre-construction phase breakdown

The pre-construction phase covers everything that must happen before construction can legally and practically begin. Each element runs in sequence or parallel with others, and the total pre-construction duration depends on which elements overlap. Here is a realistic breakdown for 2026.

Architectural design: 2-4 weeks for initial concept and developed design drawings. This includes the initial brief, site survey, concept options, client feedback and revision to produce planning-ready drawings. Complex or sensitive sites may take longer.

Planning permission: if a full planning application is required, allow 8-13 weeks from submission to decision. The statutory determination period is 8 weeks, but most London boroughs take longer due to caseload. If you are using the Larger Home Extension prior approval route, the determination period is 42 days. If your project falls within permitted development, a Lawful Development Certificate takes up to 8 weeks but you can proceed without waiting if you are confident the work is compliant.

Structural engineering: 2-3 weeks for calculations and drawings once the architectural design is finalised. The structural engineer designs the steelwork, foundations and any modifications to existing structure. This must be complete before Building Regulations submission and before the contractor can price the structural elements accurately.

Building Regulations: 2-4 weeks for a full plans application to be checked and approved. Alternatively, a building notice can be served to start work more quickly, but this carries more risk of on-site changes.

Party wall notices: 4-8 weeks from serving notice to having an award in place (if the neighbour dissents) or 2 weeks (if the neighbour consents). This can run in parallel with other activities but must be complete before construction starts.

Procurement and mobilisation: 2-4 weeks for the contractor to finalise pricing, order key materials (particularly structural steel and bespoke glazing which have 4-6 week lead times) and schedule the project into their programme.

On-site timelines by extension type

Once pre-construction is complete, the on-site build duration depends on the type and scale of the extension. Realistic 2026 on-site timelines for London and the South-East are as follows.

Single-storey rear extension (15-25 m²): 12-16 weeks. This covers groundworks and foundations (2-3 weeks), structural shell including steelwork, blockwork, roof and external envelope (4-5 weeks), first fix including electrics, plumbing and plastering (3-4 weeks), and second fix including kitchen, flooring, decoration and snagging (3-4 weeks).

Side-return extension: 10-14 weeks. Typically quicker than a standard rear extension because the footprint is smaller and the structural work is often simpler, but drainage and access constraints in the side return can add complexity.

Wraparound extension: 14-20 weeks. The combination of rear and side elements, additional steelwork at the corner junction, and larger overall footprint extends the programme. Multiple glazing systems and more complex roofing arrangements add to the timeline.

Double-storey extension: 16-22 weeks. The foundations and structural shell take longer because the loads are greater and the scaffolding and roof structure are more complex. First-fix and second-fix durations increase proportionally with the additional floor area.

Loft conversion: 8-16 weeks depending on type. Velux conversions sit at the shorter end; mansard conversions at the longer end. The programme is driven by structural complexity and the extent of roof alteration.

What causes delays and how to avoid them

The most common causes of delay on extension projects are entirely avoidable with proper planning. Late design changes are the number one cause - altering the layout, moving door positions or changing the kitchen specification after construction has started triggers a cascade of rework across every trade. Make final design decisions before construction begins.

Slow material deliveries are the second most common cause, particularly for bespoke items. Structural steel, aluminium glazing systems, roof lanterns, bespoke joinery and specialist bathroom fittings all have lead times of 4-8 weeks in 2026. Order these items at contract stage, not when the builder asks for them mid-project.

Weather affects groundworks and external envelope work. Rain slows foundation excavation and concrete pouring. Frost prevents bricklaying and plastering. Wind restricts crane operations and roof work. Projects starting in spring or early summer tend to progress more smoothly through the structural phase than those starting in autumn or winter.

Unforeseen site conditions - discovering that the ground is contaminated, that existing foundations are inadequate, that hidden drainage runs through the site, or that the existing structure is in worse condition than expected - are genuine risks. A proper pre-construction survey (including trial pits for foundation design) reduces these risks significantly but cannot eliminate them entirely. This is why we recommend a 5-10% contingency on every project.

How RCB manages your programme

Programme management is one of the most important things a contractor does, yet it is invisible to most clients until something goes wrong. At RCB Design & Build, every project has a detailed programme showing each phase, each trade, each key material delivery and each inspection milestone. This programme is shared with the client at contract stage and updated weekly during construction.

We manage lead times proactively - ordering structural steel, glazing and other long-lead items as soon as the contract is signed, not when the items are needed on site. We sequence trades carefully so that each team arrives to find the previous trade's work complete and the site ready for them. And we build weather contingency into programmes for external work phases rather than pretending it never rains in London.

Our Checkatrade reviews consistently mention programme reliability as a key strength. Delivering on time is not about rushing - it is about planning properly, procuring early, sequencing intelligently and communicating clearly when anything changes. That is what FMB and TrustMark accreditation actually means in practice: a professional approach to every aspect of the project, not just the bricklaying.