A house extension usually takes far longer from first idea to handover than the building work itself. The build is often a matter of a few months, but design, consents and neighbour notices can add several more before anyone digs a trench. This guide walks through each stage in the order it happens, with the statutory timings that apply in England and the points where projects most often slip.
TL;DR
- Expect the paperwork before construction to take roughly 3 to 6 months, covering drawings, planning (if needed) and Party Wall notices.
- A householder planning application is normally decided within 8 weeks, and building regulations approval is a separate process.
- Party Wall notices must go out at least 1 or 2 months before work, depending on the type of work.
- Mid-build changes are the biggest cause of overruns, so agree a complete, itemised schedule of works before you start.
- Plan on 9 to 12 months from first conversation to finished extension for a typical project, longer if planning or neighbour consent becomes complicated.
Table of Contents
- The whole timeline at a glance
- Stage 1: Brief, budget and survey
- Stage 2: Design and drawings
- Stage 3: Planning permission or permitted development
- Stage 4: Building regulations approval
- Stage 5: Party Wall notices
- Stage 6: Tender, contract and mobilisation
- Stage 7: The build itself
- Stage 8: Completion and sign-off
- What most often delays an extension
- Our perspective: where the time really goes
- Planning an extension in the West Midlands?
- FAQ
- Sources
The whole timeline at a glance
The table below shows the stages and a sensible planning allowance for each. The statutory periods are fixed by law. The others are working assumptions for planning your own diary, and your designer and builder should confirm them in a written programme for your specific site.
| Stage | Planning allowance | What drives it |
|---|---|---|
| Brief, budget and survey | 2 to 6 weeks | Your decisions, surveyor availability |
| Design and drawings | 4 to 10 weeks | Number of design revisions |
| Planning permission (if needed) | 8 weeks statutory, plus preparation | Local authority workload, objections |
| Building regulations | Around 5 weeks for full plans | Structural calculations, council or private approver |
| Party Wall notices | 1 to 2 months notice, longer if disputed | Neighbour response |
| Tendering and contract | 2 to 4 weeks | Clarity of the schedule of works |
| Construction | Typically a few months, longer for double-storey or wrap-around schemes | Size, weather, materials, changes |
| Final inspection and certificate | A few weeks | Notifying building control, snagging |
Add these up and you can see why a project that looks like a four-month build can run to a year or more from the first sketch. Overlap helps: building regulations drawings can be prepared while planning is being decided, and Party Wall notices can be served as soon as the design is fixed.
Stage 1: Brief, budget and survey
The first weeks decide how smooth the rest will be. Before any drawing, you need to know what you want the space to do, roughly what you can spend and what the house and garden allow.
A measured survey of the existing house is the starting point for accurate drawings. Surveyor lead times vary, so book it early rather than waiting until you have chosen a designer. This is also the moment to settle the budget with some honesty. A realistic figure at the start prevents a redesign three months later, and a redesign is a delay.
If you are weighing up what type of extension suits you, our guides on conservatory versus extension and two-storey extension rules cover the planning limits that shape the brief.
Stage 2: Design and drawings
Design usually takes several rounds. You see a first layout, comment, see a revision, and repeat until the plans are right. Each round adds days or weeks depending on how quickly feedback moves in both directions.
Two things lengthen this stage more than anything else:
- Changing the brief halfway through, such as adding a bedroom to what began as a kitchen-diner.
- Treating planning drawings and construction drawings as the same thing. They serve different purposes, and budgeting for both packs avoids a stall later. Our note on extension planning drawings explains how a single design-and-build team can reduce the duplication.
The Royal Institution of Chartered Surveyors (RICS) advises in its home extensions consumer guide that a qualified professional handling design, statutory consents and contract management lowers the risk of disputes and project failure. Time spent getting this stage right is the cheapest time in the whole project.
Stage 3: Planning permission or permitted development
Not every extension needs a planning application. In England, many extensions fall under permitted development rights if they meet size and location limits. Extensions that exceed those limits, or that sit on designated land such as a conservation area, need a householder application. The Planning Portal sets out that these are typically decided within 8 weeks, or 13 weeks for complex cases, and that permission is generally valid for three years.
Those 8 weeks only start once the council has validated your application, so allow time beforehand to prepare it and time afterwards for any request for more information. If you are unsure which route applies, it is worth checking before you commit to a design. In the West Midlands, our guides on side extension planning rules and front extensions under permitted development show where the limits typically bite. For kitchens specifically, see why planning can take 8 weeks.
If you are using permitted development, you may want to confirm it formally rather than assume. A wrong assumption discovered after the build starts can stop work entirely.
Stage 4: Building regulations approval
This is the stage homeowners most often confuse with planning. They are separate. Even when no planning permission is needed, most structural extensions still need building regulations approval, which covers structural safety, fire protection, energy efficiency and ventilation.
According to GOV.UK, you can apply through your local council or a private registered approver. A full plans application gets formal approval, usually within 5 weeks, while a building notice suits simpler, smaller projects and allows work to begin 2 days after submission. A building notice is quicker, but it gives you less certainty before the builder starts, so for anything with structural alterations a full plans application is the safer choice.
Structural calculations are often what hold this stage up. If you are choosing between construction methods, the way the structure is built also affects site time. Our comparison of timber frame versus brick looks at the difference for West Midlands extensions.
Stage 5: Party Wall notices
If your extension touches a shared wall, sits on or at the boundary, or involves digging near a neighbour's building, the Party Wall etc. Act 1996 applies. The Act itself and the government's guidance on party wall disputes set out the process. In summary, written notice must be served at least 2 months before work on a party structure, or 1 month for excavation and line-of-junction work.
If your neighbour consents, that is the end of it. If they dissent or do not reply, surveyors must be appointed to agree an award, and this is where the delay appears. We cover the sequence in our party wall process guide and explain what applies to an extension.
Two practical points:
- Talk to your neighbours before the formal notice arrives. A friendly conversation often speeds consent.
- Serve notices as soon as the design is fixed, not when the builder is ready to start. Starting the clock late is the most avoidable delay in this whole list.
A Schedule of Condition, recording the state of the neighbouring property beforehand, is strongly advised because you are liable for damage to adjoining properties.
Stage 6: Tender, contract and mobilisation
With drawings approved, you need a fixed price and a signed contract. Builders need time to price from a full set of drawings, order long-lead items such as windows, steel and bifold doors, and book their trades. Four weeks is a sensible allowance, and it can stretch if materials have long delivery times.
This is also the point to insist on an itemised, costed schedule of works. RICS notes that changes requested after work has started are typically the most expensive and time-consuming pitfalls. Decide the kitchen layout, the electrical positions, the tile choices and the heating before the first brick, not after.
Stage 7: The build itself
Once on site, construction follows a fairly predictable order:
- Site set-up, demolition and excavation, followed by the foundation inspection.
- Foundations and drainage.
- Walls, steelwork and roof structure, so the building becomes weathertight.
- First fix: electrics, plumbing and heating.
- Plastering, floors and screeds, which need drying time.
- Second fix: kitchen, sockets, sanitaryware and decoration.
Building control surveyors make inspections at key stages, for example at commencement, at foundation excavation and at completion, as shown in the inspection schedule published by the London Borough of Sutton and by Hart District Council. Your builder needs to give notice before each inspection, and waiting for an inspector can pause work if the notice is late. Inspections are periodic, so you still need good day-to-day site supervision. As Plymouth City Council explains, responsibility for compliance rests with the homeowner and builder, not the inspector.
How extension type changes the build
A single-storey rear or side-return extension is the quickest to build. A double-storey extension adds a first-floor structure, a larger roof tie-in and more internal alteration, so expect a noticeably longer programme. A wrap-around extension combines rear and side work and sits at the longer end. Over-garage extensions involve checking that the existing structure can carry the new floor above. Ask any builder for a week-by-week programme for your type of extension rather than a generic figure.
If the extension includes a new bathroom, the fit-out timing matters too. The bathroom installation timeline shows how trades and drying times stack up in a room that is fitted out in stages.
Weather and materials
Groundwork and roofing are the stages most exposed to bad weather. Winter starts can slow foundations, while a spring start usually gives you the roof on before the wettest months. Whatever the season, long-lead items ordered early protect the programme.
Stage 8: Completion and sign-off
Finishing the build is not the end. The final inspection and completion certificate are not automatic. As Sheffield City Council's guidance makes clear for full plans applications, you need to notify your building control body when the work is done. Without that certificate, you may struggle with mortgages and sales later. Do not assume your builder has dealt with it, and ask for the certificate before the final payment is released.
Building control fees vary by council and project size, and VAT at 20% applies to these services, except for regularisation certificates. The same GOV.UK page covers the detail.
What most often delays an extension
In rough order of how often they cause trouble:
- Late Party Wall notices. The notice period cannot be compressed.
- Design changes after approval. Each change can mean revised drawings, revised building regulations details and a re-pricing.
- Assuming planning and building regulations are one thing. They run on different clocks.
- Unclear schedule of works. Vague allowances for finishes lead to disputes and waiting.
- Late materials. Steel, windows and bespoke kitchens can take weeks.
- Missed inspection notices. Work cannot be covered over until it has been checked.
Our perspective: where the time really goes
When homeowners ask us how long an extension takes, they usually mean the build. In our experience of working across Birmingham, Solihull and the surrounding areas, the more useful question is how long before the build starts, because that is where the surprises sit.
Our approach is one coordinated team from design to handover. That matters for time because design, planning, building regulations and construction are not handed between companies that each wait on the other. We also give you a realistic budget before any site survey, so the scope you design is the scope you can afford, and the drawings do not need to be redone halfway through.
We handle the planning and building regulations paperwork for you. We would still urge you to talk to your neighbours early and to make your decisions on finishes before building starts. Those two habits save more weeks than any amount of site pace.
— Raja
Planning an extension in the West Midlands?
If you want a realistic programme for your own home, we can talk you through it. We design and build single-storey, double-storey, kitchen, rear, side-return, wrap-around and over-garage extensions under one team, with planning, building regulations and project management included. See our extension services or read our guide to extension costs in the West Midlands to start with a budget.
FAQ
How long does a house extension take from start to finish?
Allow roughly 9 to 12 months from first conversation to completion for a typical project. The design and consents phase can take 3 to 6 months before building starts. Complex sites, neighbour disputes or double-storey schemes can take longer.
Do I need planning permission for a house extension?
Not always. Many extensions fall under permitted development if they stay within size and location limits, but conservation areas and larger schemes need a householder application. Building regulations approval is needed either way for most structural work. Check with the Planning Portal or your local council before you fix a design.
Can I live in the house while the extension is built?
Many people do, particularly with a rear or side-return extension, though kitchen work usually means a temporary kitchen elsewhere. Disruption is greatest during demolition, structural work and the connection to the existing house. Ask your builder how and when services will be isolated.
What is the quickest way to shorten the timeline?
Fix the design and finishes early, serve Party Wall notices as soon as the plans are settled, and run building regulations drawings alongside planning. Using one team for design and build also removes handover gaps. Rushing the build itself rarely helps and tends to cost more.
Sources
- Planning Portal: planning permission and building regulations for extensions
- GOV.UK: building regulations approval, how to apply
- Party Wall etc. Act 1996
- GOV.UK: preventing and resolving disputes in relation to party walls
- RICS: home extensions consumer guide
- Plymouth City Council: building control
- Sheffield City Council: full plans building regulation approval
- London Borough of Sutton: inspection schedule
- Hart District Council: site inspections