If you want builders in your garden by March, the work starts this month. The digging can wait. The drawings, the planning decision and the party wall notices can't, because each one runs on a clock you don't control. People often ask us when to plan an extension, and for a spring start our answer is October. Here's the calendar worked backwards from a March start, what can overlap, and what happens if you leave it until January.
When to plan an extension for a spring start: why October
Before anyone digs a trench, an extension has to clear several sets of rules, and each one has a waiting period built in.
- Planning. A householder application costs £548 in England and should be decided within 8 weeks. The clock starts when the council validates the application, not when you send it (Planning Portal).
- Building regulations. A full plans application must get a decision within five weeks, or up to two months if you agree (Planning Portal).
- Party wall. Two months' notice for work to a party wall. One month for a new wall on the boundary line or for digging near next door's foundations (GOV.UK explanatory booklet).
- Sewers. If a public sewer runs within three metres of the new walls, you'll need to check Wessex Water's criteria, and building over one needs their approval before work starts (Wessex Water).
Add the design stage in front of all that, plus Christmas, which takes the best part of a fortnight out of most diaries. You're looking at four to five months of paperwork before a spring start. Begin in October and that lands in March.
The back-planned calendar for a March start
This assumes an extension that needs full planning permission and shares a wall with next door, which is common on Bristol terraces and semis.
| When | What happens | Who does it |
|---|---|---|
| Early October | Brief, measured survey, choose a designer | You and your architect |
| October to mid-November | Planning drawings | Your architect |
| By mid-November | Planning application submitted (£548) | You or your architect |
| Mid-November to mid-January | Eight-week decision period, from validation | Your local council |
| November to December | Structural calcs (usually £500 to £1,000), building regulations drawings, drain survey, builders' quotes | Engineer, architect, you, builders |
| Early January | Party wall notices served | You |
| January | Building control application | You, or us for a small admin fee |
| January to February | Build-over approval, if a public sewer is close | You and your architect, with Wessex Water |
| February | Detailed, itemised quote agreed, start date booked | You and us |
| March | On site | Us |
A few notes on that table. The drain survey is something you arrange. We don't CCTV drains, but we need the result before we can price the groundworks properly. Party wall notices are served by you as the owner, not by your builder, and our party wall page explains what goes in them. For building control, you normally make the application, and we liaise with the inspector and book the inspections.
If you don't have an architect yet, our design and build service works with partner architects, so the drawings and the build price develop together. Planning a second storey? That usually means a full planning application, so the October start matters even more. Our two-storey extensions page has more on how we build them.
What can run in parallel, and what can't
Some stages overlap happily. Others have a fixed order.
These can overlap:
- Structural calculations and building regulations drawings, while the planning decision is pending.
- Builders' quotes, priced from the planning drawings. Ours are detailed and itemised, and the price only changes if you agree the change first.
- Party wall notices, once the design is settled. You don't have to wait for the planning decision to serve them.
These can't be rushed:
- Validation. The planning clock doesn't start until the council validates the application. A missing drawing or the wrong fee sends it back.
- Notice periods. You can start sooner than the party wall notice allows only with your neighbour's written agreement. If they don't respond within 14 days, a dispute is assumed and surveyors get involved, which adds weeks.
- Steel sizes. They depend on the final layout. Calcs can start while planning is pending, but if the planner asks for a smaller footprint or a different roof, the engineer has to rerun them. Allow a little slack for that.
What happens if you start in January instead
Run the same steps from January and every date slides:
- Survey and drawings run from January into February.
- The planning application goes in late February, with a decision around late April.
- Party wall notices served in March run until May at the earliest.
- Building control and quotes follow through the spring.
- You're on site in May or June at the earliest, and later if the builder you want is booked.
So the extension you wanted finished for summer is now being built through it, and the garden you meant to use is a compound of skips and block stacks. Builders' diaries fill up too. We explained why in why good builders in Bristol have waiting lists.
Winter is still a building season
If you already have planning permission or a lawful development certificate, you don't need to wait for spring. Foundations, drainage and blockwork carry on through a Bristol winter with the right precautions. Our Sherwell Road new build started its excavation, foundations and drainage in February, in winter ground. The clocks go back on Sunday 25 October, but our site hours stay 8am to 5pm, Monday to Friday.
Whatever the season, plan the disruption as carefully as the paperwork. Decide where the skip and materials will go, which weeks the kitchen will be out of action, and how you'll get in and out while the back wall is open. Our guide to living at home during a build covers the dust and noise side. For a sense of scale, a conservatory-to-kitchen extension we built in Kingswood took 12 weeks on site, start to finish.
Frequently asked questions
When should I start planning an extension for a spring build?
Start in October if you want builders on site in March. Survey and drawings take most of October and early November, planning takes eight weeks from validation, and party wall notices need one or two months. Christmas slows everyone down. Starting later usually pushes the build into summer.
How long does planning permission take for an extension?
Councils should decide householder applications within 8 weeks, counted from validation rather than submission. The fee in England is £548. Allow time before that for the drawings and for validation, and after it for any planning conditions you need to deal with before work starts.
Do I need planning permission or a lawful development certificate?
If your extension fits permitted development, you don't need planning permission, but a lawful development certificate proves it. It costs £274 for a proposed extension and should be decided within 8 weeks (Planning Portal). Permitted development limits single-storey rear extensions to 3 m deep on terraced and semi-detached houses, and 4 m on detached ones.
When do I need to serve party wall notices?
Two months before work to a party wall, and one month before building on the boundary line or digging near next door's foundations. You serve them as the owner. Your neighbour has 14 days to respond, and you can only start sooner with their written agreement.
Who applies for building control approval?
You normally make the building control application. We liaise with the inspector, book the inspections at each stage, and can make the application for you for a small admin fee. A full plans application gets a decision within five weeks, or up to two months if you agree.
We have capacity this winter. Let's talk before it's gone. Tell us what you're planning and Dan or Craig will call you back within one working day.
