How to Plan a Home Extension in the UK
Where to actually start, whether you need planning permission, and the realistic timeline from first sketch to first brick — not the version where everything happens in six weeks.
Where to start with a home extension
Before anyone draws a single line, get clear on what the space is actually for — a bigger kitchen-diner, a home office, an extra bedroom. That brief shapes everything after it. From there:
- Commission a measured survey of your existing house — the accurate starting point every drawing depends on
- Get architectural drawings done — not legally required, but the drawings are what your planning and Building Regs applications are built on, so this isn't the place to freehand it
- Set a realistic budget before you fall in love with a design — see our home extension cost guide for current £/m² rates
Do you need planning permission?
This is the fork in the road that decides your entire timeline. Three routes, depending on size and house type:
Permitted Development
- Single-storey rear, up to 3m (terraced/semi) or 4m (detached)
- Still needs Building Regs sign-off
- ~75–80% of extensions fall here
Larger PD (Prior Approval)
- Up to 6m (terraced/semi) or 8m (detached)
- Neighbours get 21 days to object
- Council must decide within 42 days
Full Planning Permission
- All two-storey extensions
- Conservation areas, listed buildings
- Anything beyond Larger PD limits
Planning routes compared
| Route | Size limit | Cost | Decision time |
|---|---|---|---|
| Permitted Development | 3m (terraced/semi) / 4m (detached) | £0 (application) | N/A — no application |
| Larger PD (Prior Approval) | 6m (terraced/semi) / 8m (detached) | £249 | Up to 42 days |
| Full planning permission | No fixed limit (subject to approval) | ~£258+ (standard household fee) | 8–12 weeks realistic |
| Lawful Development Certificate (optional) | N/A — confirms PD status | ~£103 | 4–6 weeks |
The planning process, step by step
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1. Define your brief and budget
What the space is for, roughly what you can spend. Everything else is downstream of these two answers.
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2. Measured survey and drawings
An architect or architectural technician produces the drawings your planning and Building Regs applications will be based on.
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3. Establish your planning route
Permitted Development, Larger PD Prior Approval, or full planning permission — confirmed against your specific house and area, not just the general size limits.
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4. Structural engineer input
Needed for Building Regulations regardless of your planning route — this is where beam sizes and foundation depths get specified.
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5. Serve party wall notices
At least 2 months before work starts if a shared wall is affected, or 1 month before excavation near a neighbour's foundations. Run this in parallel with planning, not after.
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6. Submit your Building Regulations application
Required even when no planning application is needed — this is the system that checks the extension is actually safe to build.
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7. Get quotes and choose a builder
With drawings and approvals in hand, quotes are finally comparing like for like instead of guesswork.
Realistic timeline
A single-storey extension typically takes 3 to 5 months of preparation before construction even starts, with the stages overlapping rather than running end to end:
| Stage | Typical duration |
|---|---|
| Design and drawings | 4–8 weeks |
| Planning decision (if needed) | 8–12 weeks |
| Party wall notice period (runs in parallel) | 2–3 months |
| Construction (single-storey) | 8–12 weeks |
The most common causes of delay: planning taking longer than the council's own target, unforeseen ground conditions once digging starts, mid-build design changes, and party wall disputes with a neighbour who's taking their time.
Local considerations for Leicestershire homes
Most homes across Hinckley and the wider Leicestershire area sit outside conservation areas, so standard Permitted Development limits usually apply without complication — but it's always worth a quick check with the local planning authority before assuming, since Article 4 directions (which remove PD rights locally) exist in pockets across most councils. 1930s semis typically have straightforward rear gardens well suited to a standard single-storey PD extension; Victorian terraces are more likely to need a side return, which brings a party wall agreement into the mix regardless of which planning route applies.
How to avoid delays
- Confirm your planning route with the council before finalising a design, not after
- Serve party wall notices as soon as drawings are settled, not once building work is about to start
- Get a structural engineer involved early — redesigning around a beam size late in the process costs real time
- Don't book a builder's start date until Building Regs approval is actually in hand
Already know your budget range? See our home extension cost guide for current £/m² prices. For Renofox's own extension service across Hinckley and Leicestershire, visit our Home Extensions page. Considering a loft conversion instead? That's a different planning path entirely.
Frequently asked questions
Where do I start with a home extension?
Start with a clear brief of what you actually need the space for, then commission a measured survey and architectural drawings. You don't legally need an architect, but one is strongly recommended for most extensions, since their drawings are what your planning and Building Regulations applications are built on.
Do I need planning permission for a home extension?
Around 75 to 80% of extensions fall under Permitted Development and only need Building Regulations sign-off, not a planning application. Single-storey rear extensions up to 3 metres (terraced/semi-detached) or 4 metres (detached) are standard Permitted Development. Larger single-storey extensions, up to 6 or 8 metres, need a Prior Approval application instead. Two-storey extensions always need full planning permission.
How long does it take to get planning permission for an extension?
A standard planning application decision takes up to 8 weeks, though 8 to 12 weeks is more realistic once validation and any revisions are factored in. A Prior Approval application for a larger Permitted Development extension must be decided within 42 days. The whole preparatory phase, from first architect meeting to breaking ground, typically takes 3 to 5 months.
What is a Lawful Development Certificate and do I need one?
A Lawful Development Certificate is a formal council document confirming your extension qualifies as Permitted Development. It isn't mandatory, but it costs around £103, takes 4 to 6 weeks, and is useful evidence if you sell the house later or a neighbour challenges the work.
When do I need to serve a party wall notice?
At least two months before work starts if it affects a shared wall, or one month before any excavation near a neighbour's foundations. The Party Wall Act is entirely separate from planning permission and Building Regulations, so it needs handling in parallel, not after everything else is sorted.
What's the difference between planning permission and Building Regulations?
Planning permission controls what the extension looks like and how it affects neighbours and the street. Building Regulations control how it's actually built, structurally and safely. Most extensions need Building Regulations approval even when no planning application is required.
Ready to talk through your plans?
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