Best time of year to start an extension

1. What does the Warwickshire weather record actually show?

At the Met Office Wellesbourne station, 41 of the 47 average yearly air frost days fall between November and March, while monthly rainfall only moves between about 40 and 63 millimetres.

The Met Office publishes 30-year averages for its Wellesbourne station in Warwickshire. For 1991 to 2020 it records an average of 47.3 days of air frost a year. December has about 10, January about 10, February about 9, March about 7 and November about 5. June has almost none, and July to September have none at all. Rugby is not Wellesbourne, so treat these as a county guide rather than a forecast for your street.

Rain is a different story. The wettest month on the same record is October at about 63 millimetres and the driest are February and March at about 40. Days with at least a millimetre of rain range from about 8.5 in July to about 12 in November. Summer has slightly fewer rain days, but not less rain overall. What really changes through the year is frost and sunshine: average sunshine is about 54 hours in December and about 200 hours in July.

Check the guidance: Met Office: Wellesbourne climate averages, MyJobQuote: reasons to start an extension in summer, MyJobQuote: when is the best time to build an extension?, Less: best time to get home improvements done.

2. Which stages of an extension depend on the weather?

LABC treats 2°C as the minimum working temperature for most materials, so frost matters most while the foundations, walls and roof are open to the air.

An extension is weather sensitive in its first half and much less so once it is weathertight. Digging and pouring foundations, laying bricks and blocks, and getting the roof covered all happen outside. After the roof is on and the openings are closed, most of the remaining work happens indoors: first fix services, plastering, screeding, second fix and decorating.

Building control guidance from LABC, dated February 2021, says two degrees is the minimum working temperature for most materials. New masonry needs frost protection when temperatures drop below 2°C, render should only go on at 2°C and rising, and plastering and screeding should not go ahead unless the building is free from frost. It also says frozen materials should not be used at all, and that bricks, blocks, sand and cement should be kept covered.

Rain matters too, but in a different way. A wet spell slows digging and brickwork and can leave a site muddy, as the lead photograph shows. The averages above suggest that in Warwickshire rain arrives in every season, so a programme should allow for wet days whatever month it starts.

Check the guidance: LABC: working in cold weather.

3. How long does the paperwork take before work starts in Rugby?

Most planning applications are decided within 8 weeks, a full plans building regulations decision takes up to 5 weeks or 2 months, and party wall notice must be served at least 2 months ahead.

Weather decides how easily the build runs. The paperwork decides when it can start. GOV.UK says most planning applications are decided within 8 weeks, and that unusually large or complex applications in England have 13 weeks. Rugby Borough Council says permitted development rights can be adapted or removed, and can differ for listed buildings and conservation areas. The council lists 19 conservation areas in the borough, where it has extra powers, and charges a fee for pre-application advice.

If your extension falls within permitted development, the Planning Portal limits for a single-storey rear extension include a depth of 4 metres beyond the original rear wall for a detached house and 3 metres for other houses, a height of 4 metres, and eaves of no more than 3 metres within 2 metres of a boundary. Outside designated land, larger rear extensions of up to 8 metres for a detached house or 6 metres for others need a prior approval application. A lawful development certificate is optional, but it confirms the position in writing, and the council has eight weeks to decide.

Building regulations are a separate approval. GOV.UK says a full plans application should be decided within 5 weeks, or 2 months if you agree. With a building notice, work can start 2 days after the notice is submitted, but there is no formal approval of plans. In Rugby, building control is run by Warwick Building Control, a partnership between Rugby Borough Council and Warwick District Council. Party wall duties are separate again: notice must be given between 2 months and a year before work starts.

Check the guidance: GOV.UK: planning permission, Rugby Borough Council: Do I need planning permission?, Rugby Borough Council: conservation areas, Planning Portal: extension planning permission, Planning Portal: lawful development certificates, GOV.UK: how to apply for building regulations approval, Rugby Borough Council: Warwick Building Control, GOV.UK: party walls and building work.

4. Working back from the start date you want

For a spring start, the design, approvals and builder selection usually need to be under way in the autumn or winter before.

The Federation of Master Builders (FMB) guide to extensions says a straightforward single-storey extension may take around three to six months to build. Before that, it lists surveys and design, planning or prior approval, building regulations drawings, structural calculations, party wall notices, choosing a contractor, ordering long lead products and the builder's existing commitments. Older FMB guidance, dated January 2019, said homeowners should expect to wait at least four months for a quality builder to be available.

Put those together and work backwards. Suppose you want a builder on site in April. Party wall notice has to be served by early February at the latest. If planning permission is needed, an application validated in early December could be decided in early February, assuming no extension of time. That leaves design and drawings to be finished in November. This is our arithmetic on the published periods, not a promise about any council, builder or project.

If the design is not ready, choosing a start month will not help much. The month you start matters less than whether the approvals, drawings and contract are settled before the first day on site.

Check the guidance: FMB: house extensions, the ultimate guide, FMB: how far ahead to plan home improvements.

5. Season by season: what each start gives you

Each start season has a trade off between site conditions, finish date and the time left to prepare, so pick the one that suits your household and your paperwork.

A Wickes trade page, undated, says larger extensions usually start in spring because the ground is soft enough to dig. That fits the frost record, but it is a retailer's view, not a rule. The table sets out the trade offs using the sources above, rather than a ranking.

Check the guidance: Wickes: top spring jobs for trades, Met Office: Wellesbourne climate averages, LABC: working in cold weather.

6. Nesting birds, VAT and household timing

All wild birds are protected under the Wildlife and Countryside Act 1981, so clearing a nest that is in use or being built is an offence whatever the build programme says.

GOV.UK planning guidance, updated April 2025, says it is an offence to take, damage or destroy a nest that is in use or being built. It suggests avoiding harm by doing work at a different time of year or removing habitat features before the nesting season. If your extension needs a hedge, shrubs or a tree cleared, or roofing work near existing eaves, ask early whether that clearance should happen before the season starts.

Timing does not change the VAT rate. GOV.UK says most building work on existing houses is standard rated at 20%, with certain exceptions. When you compare quotations, check whether each figure includes VAT, whatever month the work is planned for.

Then there is your own calendar. An extension at the back of the house often means losing part of the garden and sometimes the kitchen for a period. Think about school holidays, family events and Christmas, and ask your builder how the programme treats the weeks when the inside of the house is affected.

Check the guidance: GOV.UK: wild birds advice for planning decisions, GOV.UK: VAT for builders.

7. What to bring to the first conversation

A builder can only talk usefully about a start date once they know the design stage, the approvals you need and any boundary issues.

Bring your Rugby postcode, photographs of the rear of the house and the garden, any drawings, and whether you think the work is permitted development or needs a planning application. Say whether a neighbour's wall or foundations are within reach of the work, since that brings party wall notice into the plan, and whether the house is in a conservation area. Tell the builder the season you would prefer and why. A good answer will explain what needs to happen first and when, rather than simply naming a date.

Check the guidance: Rugby Borough Council: conservation areas, GOV.UK: party walls and building work.