How Long Does a House Extension Take in London?

A house extension in London typically takes 6–12 months from first contact to completion. The construction phase itself is shorter — a single-storey rear extension takes 12–16 weeks on site. However, design, planning permission and building regulations approval typically add 3–6 months before a single brick is laid.

Understanding the full timeline helps you plan your life around the project, set realistic expectations with your family, and make informed decisions about when to start. This guide walks through every stage in detail.

House Extension Timeline at a Glance

Stage	Single Storey	Double Storey	Notes
Design and drawings	4–8 weeks	6–10 weeks	Architect prepares plans
Planning application (if needed)	8–10 weeks	8–10 weeks	Local authority determination
Building regulations	2–4 weeks	3–5 weeks	Can run concurrently with planning
Party wall process	2–8 weeks	2–8 weeks	If applicable
Construction on site	12–16 weeks	18–24 weeks	On-site build
Total	6–10 months	8–14 months	From first meeting to handover

Stage 1 — Design and Drawings (4–8 Weeks)

The design stage is where your extension takes shape on paper. It is the most important stage of the process: time spent getting the design right before construction begins saves considerable time, cost and frustration later.

A typical design process for a house extension involves:

• Site survey and measured drawings of the existing house (1–2 weeks). An accurate survey is essential for producing compliant planning and building regulations drawings.
• Design concept and options. Your architect or design-and-build contractor (such as Alstruct) will develop layout options and discuss them with you.
• Developed design and planning drawings. Once the concept is agreed, the architect produces scaled plans, elevations and sections for planning submission.
• 3D visualisations (optional but strongly recommended). These allow you to see the finished extension before any work begins and often reveal design refinements worth making at the drawing stage rather than on site.

Double-storey extensions take longer to design because there are more rooms and more complex structural interactions to resolve. Wraparound extensions — combining rear and side elements — also take slightly longer than straightforward rear extensions.

At Alstruct, our in-house architectural team handles the full design process. You meet with our architect at the start, and we manage everything from survey through to planning submission — typically within 4–6 weeks for a standard single-storey rear extension.

Stage 2 — Planning Permission (8–10 Weeks If Required)

Not all extensions require planning permission. Many single-storey rear extensions fall within Permitted Development Rights and can proceed without a planning application. For those that do require planning permission, the process adds 8–12 weeks to the programme.

The statutory determination period is 8 weeks from the date the application is validated by the council. In London, most boroughs take 10–12 weeks. Challenging applications in conservation areas, or those that attract neighbour objections, can take 14–16 weeks or more.

Key things that can slow planning down:

• An incomplete application that is delayed at the validation stage.
• Neighbour objections that require the planning officer to respond formally.
• Conservation area applications requiring input from the council's heritage team.
• Officer requests for additional information or design amendments.
• The application being referred to the planning committee rather than determined by officers under delegated authority.

Working with an experienced London builder who understands borough-specific planning requirements significantly reduces the risk of delays. Alstruct has experience submitting and managing planning applications across all major London boroughs and can pre-empt the most common reasons for delay.

Stage 3 — Building Regulations Approval

Building regulations are separate from planning permission and govern the technical standard of construction — structural integrity, fire safety, insulation, drainage, ventilation and so on. All house extensions require building regulations approval.

There are two routes to building regulations approval:

• Full Plans application: You submit detailed structural and technical drawings to the local authority's Building Control department (or a private Approved Inspector) before work starts. This provides more certainty, takes 5–8 weeks for approval, but can run concurrently with planning permission, so it need not add to the overall timeline.
• Building Notice: A simpler notice that allows work to start quickly, with the Building Inspector inspecting at key stages during construction. This is faster to initiate but provides less upfront certainty and is generally recommended only for straightforward projects.

At Alstruct, we use the Full Plans route as standard because it provides complete certainty before construction starts and minimises the risk of disputes with Building Control during the build. We handle the submission, liaise with the structural engineer, and manage the inspections schedule throughout the project.

Stage 4 — Party Wall Process (If Applicable)

If your extension involves work on or near a shared wall with a neighbour, you are required to serve notice under the Party Wall etc. Act 1996. This applies to most single-storey rear extensions in London that use the existing rear wall of the house as part of the new structure, or that involve excavation within 3 metres of a neighbour's foundations.

The timeline for the party wall process depends on your neighbour's response:

• If your neighbour consents in writing within 14 days of receiving the notice, you can proceed immediately. This adds minimal delay.
• If your neighbour does not respond or dissents, a party wall surveyor must be appointed and a Party Wall Award produced. This typically takes 4–8 weeks, depending on the surveyor's availability and the complexity of the award.

Party wall notices should be served as early as possible — ideally at the same time as the planning application is submitted. Serving them early removes the risk of party wall process delaying the start of construction after planning consent is granted.

Stage 5 — Procurement and Pre-Construction (2–4 Weeks)

Once planning permission and building regulations approval are secured, there is typically a 2–4 week period before construction starts on site. During this time:

• Final material selections are confirmed.
• Long-lead materials (e.g., bespoke windows, roof lanterns, structural steelwork) are ordered.
• The construction programme is finalised and issued to you.
• The site is set up and prepared for work to begin.
• Any party wall matters are resolved if not already complete.

Stage 6 — Construction on Site

The construction phase is what most homeowners focus on — and it is, in many ways, the most straightforward part of the process once planning and building regulations are in place. A well-managed extension builds in a logical sequence, with each trade following the one before it.

Demolition and Groundworks (Weeks 1–3)

Construction begins with any demolition of existing structures (outbuildings, old extensions, boundary walls) and then proceeds with excavation for the new foundations. In London, the ground conditions vary widely — made-up ground, clay, London Clay and previously disturbed ground are all common. If unexpected ground conditions are encountered, additional foundation work may be required, which can add 1–2 weeks at this stage.

The groundworks team installs the foundation concrete, drainage connections, and any below-slab services. This stage is often disruptive to the garden but has minimal impact on the occupied house.

Structural Frame and Walls (Weeks 3–7)

Once the foundations have cured (typically 48–72 hours), masonry walls are built up from the foundations. For extensions incorporating structural steelwork — typically needed to create wide open spans with bifold doors or large kitchen islands — the steelwork is fabricated off-site and installed once the walls reach the correct height. The structural walls and any RSJ (steel beam) work above new openings are all completed at this stage.

Roof and Weathertight (Weeks 6–9)

Making the structure weathertight is a critical milestone — once the roof is on and the extension is watertight, internal trades can begin regardless of weather conditions. Flat roofs (typically warm-deck build-ups with GRP or EPDM membrane) are quicker to construct than pitched roofs, but pitched roofs are often more appropriate in conservation areas and for double-storey extensions.

Windows and external doors are installed at this stage, completing the weathertight envelope of the extension.

First Fix M&E (Weeks 8–11)

With the structure weathertight, mechanical and electrical first fix begins. This includes the installation of electrical conduit, cable runs, soil and waste pipes, water supply pipework, underfloor heating pipes (if applicable) and any MVHR (mechanical ventilation) ducting. First fix work runs ahead of plastering — all services must be in place and tested before walls are boarded and plastered.

Plastering and Second Fix (Weeks 10–14)

Walls and ceilings are boarded with plasterboard and either skimmed or dry-lined. This is one of the most labour-intensive stages of the build. Once plastering is complete, the extension needs to dry out properly — typically 2–4 weeks depending on the weather and ventilation — before decorating and second fix can proceed.

Second fix M&E runs concurrently with plastering drying: sockets, switches, light fittings, radiators, taps and sanitary ware are all installed during this period. Kitchen and joinery installation also typically takes place in weeks 12–14.

Finishing and Decoration (Weeks 13–16)

The final weeks of a single-storey extension project involve: floor finishes (tiles, engineered timber, polished concrete), decoration (paint, tiling), external works (patio, landscaping, boundary walls) and the critical 'break-through' — the opening up of the existing house wall to connect the new extension to the existing kitchen or living space. The break-through is planned carefully to minimise disruption and is typically the last major structural operation, taking 1–3 days.

After break-through, the project moves into snagging, final inspections by Building Control (who issue the Completion Certificate), and handover.

What Causes Delays on House Extension Projects?

Most delays on house extension projects fall into a small number of categories:

• Planning permission delays. London boroughs are consistently slower than the 8-week statutory target, and conservation area applications regularly take 14–16 weeks. Serving all consultees simultaneously and submitting a complete, compliant application from the outset are the most effective mitigations.
• Unforeseen ground conditions. Encountering old foundations, contaminated ground, high water tables or unexpectedly poor bearing capacity requires additional engineering work. A thorough site investigation before construction starts reduces (but cannot eliminate) this risk.
• Design changes during construction. Changes made once work is on site are the single biggest controllable cause of delay and cost overrun. Every change to the design, specification or layout during construction generates rework, affects the programme, and typically costs significantly more than it would have at the design stage. Finalise all design decisions before construction begins.
• Material lead times. Bespoke items — large roof lanterns, steel-framed bifold doors, specific brick matches for conservation area work — can have lead times of 6–12 weeks. Order these as early as possible, ideally before construction starts.
• Party wall disputes. If a neighbour instructs their own surveyor under the Party Wall Act, the process of agreeing an award can take several weeks. Serving notices early and maintaining good communication with neighbours reduces this risk.
• Subcontractor availability. Quality tradespeople in London — particularly structural engineers, specialist plasterers and bespoke joinery manufacturers — can have busy schedules. Working with a builder who has established relationships with reliable subcontractors is important.

How to Speed Up Your House Extension Project

There are several practical steps that can materially reduce the overall programme:

1. Engage an architect or design-and-build contractor early. The sooner design work begins, the sooner planning can be submitted. Waiting until spring to start thinking about a summer project is a common mistake — planning alone takes 10–12 weeks.
2. Use permitted development where possible. If your extension qualifies as permitted development (or can be designed to do so), you save 10–12 weeks on the planning stage. A Lawful Development Certificate can be applied for concurrently with building regulations preparation.
3. Serve party wall notices immediately. As soon as your design is sufficiently developed, serve party wall notices. The 14-day consent period starts immediately, and if your neighbour consents, there is no further delay.
4. Run building regulations concurrently with planning. There is no reason to wait for planning consent before submitting Full Plans to Building Control. If the planning application results in design changes, you can amend the building regulations application accordingly.
5. Order long-lead items early. Once the design is agreed, identify any items with long lead times and order them immediately — before planning consent is granted if possible. If planning is refused, cancellation fees are generally modest compared with programme delays.
6. Choose a contractor with capacity. The best London extension builders are typically booked 2–4 months in advance. Start tendering early and select your contractor before planning consent is granted, so construction can begin as soon as consent is received.

How Alstruct Manages Project Timelines

At Alstruct, programme management is central to our approach. Before work begins, we produce a detailed construction programme — a Gantt chart showing every stage of the project, the sequence of trades, key milestones and the date we expect to hand over to you. We share this programme with you at the outset and update it throughout the project.

Our in-house team handles design, planning and construction, which eliminates the coordination delays that arise when different firms are responsible for different stages. Our project managers monitor progress daily against programme, anticipate bottlenecks before they materialise, and keep you informed at every stage.

We have successfully delivered single-storey rear extensions in London within 14 weeks on site and completed complex double-storey and wraparound projects within 22 weeks. Our track record of on-programme delivery is one of the reasons homeowners across London choose Alstruct for their extension projects.

See our completed extension projects, including our Evelyn Road renovation and extension and Lyal Road renovation in Bow, for examples of our work. Learn more about our house extension services.

Frequently Asked Questions