Key takeaways
- A straightforward single-storey extension typically takes around three to six months to construct once work starts on site.
- Double-storey extensions usually run longer because of the extra structural work, scaffolding and second-floor fit-out.
- Planning permission for a householder extension carries a statutory eight-week decision period in England, and this happens before any building begins.
- Weather, ground conditions, hidden defects and mid-build design changes are the most common causes of delay.
- A clear build programme and an experienced local contractor are the best protection against your project overrunning.
Most single-storey house extensions take around three to six months to build once work starts on site, while double-storey extensions usually take longer because of the extra structural and fit-out work. How long an extension takes depends on its size, the ground beneath it, the weather and how smoothly planning and preparation go before the first brick is laid.
This guide walks you through realistic timelines, the stages of a build week by week, and the factors that speed a project up or slow it down. If you are planning work in Northampton or the wider county, it will help you set expectations and plan around the disruption.
Typical timelines for single and double-storey extensions
For a straightforward single-storey rear or side extension, the Federation of Master Builders notes that the build itself may take around three to six months to construct. That window covers everything from digging the foundations to finishing the interior.
Double-storey extensions typically take longer. The extra floor means more foundations work in some cases, a full second level of structure, more scaffolding time and a larger area to plaster, wire and decorate. As a rough rule, a two-storey project often runs four to six months or more on site, depending on complexity.
A few things pull these numbers around:
- Size and layout. A small utility-room extension is quicker than a large open-plan kitchen and dining space.
- Specification. Bespoke glazing, structural steel and complex rooflines add time.
- Internal remodelling. Knocking through into existing rooms and rerouting services extends the programme.
- Site access. Tight access for deliveries and machinery slows the early stages.
Remember that the build is only part of the story. Design, planning and preparation often take several months before anyone arrives on site. From first enquiry to finished room, many homeowners should budget six to twelve months in total.
The stages of an extension build week by week
Every project is different, but most single-storey extensions follow a similar rhythm. Here is a typical sequence to give you a feel for how the weeks unfold.
Weeks 1 to 2: Setting up and groundworks. The team arrives, sets up welfare facilities and site protection, then starts excavation. Foundations are dug, inspected and poured. The type of foundation makes a real difference here. Our guide to the types of house foundations explains why some sites need deeper or wider footings than others.
Weeks 3 to 4: Below ground and the floor slab. Drainage is laid and connected, the ground floor slab is built up and any below-ground blockwork goes in. This stage is sensitive to wet weather and ground conditions.
Weeks 5 to 7: Walls and roof structure. The brick and block walls rise to roof level. Lintels, steel beams and the roof structure follow. Once the roof is on and the building is weathertight, the project becomes far less exposed to the elements.
Weeks 8 to 10: First fix and making watertight. Windows and external doors go in. Inside, the first fix of electrics, plumbing and carpentry runs through the walls and floors before they are closed up.
Weeks 11 to 13: Plastering and second fix. Walls and ceilings are plastered and left to dry. Second fix work follows: sockets, switches, radiators, sanitary ware, skirting and internal doors.
Weeks 14 onwards: Finishing and handover. Decorating, flooring, kitchen or bathroom installation, snagging and a final clean bring the project to completion.
A double-storey build follows the same logic but repeats the structural and fit-out stages for the upper floor, which is why it adds weeks rather than days.
Factors that can speed up or delay your project
The FMB is clear that even a well-run build can be affected by weather, hidden defects, delayed products, and client changes. Those four causes account for most overruns, so it is worth understanding each one.
Design changes mid-build. Changing your mind about the kitchen layout or adding a rooflight after work has started is one of the biggest causes of delay. Every change ripples through the programme.
Material lead times. Bespoke windows, structural steel and specialist units can take weeks to arrive. Ordering early keeps the build moving.
Hidden problems. Old drains, poor existing foundations or unexpected ground conditions can surface once digging begins. These are hard to predict but easier to absorb when your contractor builds some contingency into the plan.
Decisions and sign-offs. Slow decisions on finishes, or delays agreeing variations, stall progress just as surely as bad weather.
Good preparation shortens the whole timeline. Finalising your design, confirming your specification and ordering long-lead items before work starts are the simplest ways to keep a project on track.
How weather and ground conditions affect timing
The early stages of a build are the most exposed. Digging foundations, laying drainage and pouring concrete all depend on reasonable ground conditions. Heavy or sustained rain can flood trenches, make the ground unworkable and push concrete pours back by days.
Winter brings shorter working days and the risk of frost, which can stop certain work altogether. This matters in Northamptonshire, where a wet autumn or a cold snap can interrupt groundworks.
Ground conditions themselves are just as important. Clay soils move with moisture, high water tables complicate excavation, and nearby trees can dictate deeper foundations. A thorough site assessment before work begins helps the team anticipate these issues rather than discover them halfway through a dig.
Once the structure is weathertight, the weather matters far less. Getting to that watertight stage efficiently is one of the main reasons an experienced contractor sequences the early weeks so carefully.
Planning around disruption while living on site
Most homeowners stay put during an extension, so planning around the disruption makes the whole experience easier.
- Expect noise and dust during the structural phase. The loudest weeks are groundworks, demolition and any knocking-through. Seal off the work zone with dust sheets and temporary partitions.
- Plan for a temporary kitchen. If you are extending or remodelling the kitchen, set up a basic cooking space elsewhere in the house for a few weeks.
- Agree site hours and access up front. Knowing when the team will arrive and where materials will be stored helps your household settle into a routine.
- Keep communication open. A short weekly catch-up with your builder keeps you informed and lets you flag decisions early, before they hold up the build.
- Protect parking and deliveries. Extensions generate regular deliveries, so a clear plan for where lorries unload avoids friction with neighbours.
A little preparation turns months of building work into a manageable routine rather than a daily surprise.
How AAS Group keeps projects on schedule
At AAS Group, we build extensions across Northampton and Northamptonshire, and keeping to schedule starts long before we arrive on site. We agree a clear build programme with you, confirm the design and specification early, and order long-lead materials ahead of time so nothing stalls the work.
Our groundworks experience matters here. Because we handle the foundations and drainage ourselves, we can read a site accurately, allow for local ground conditions and get to a weathertight structure efficiently. That early momentum protects the rest of the timeline.
We also keep you informed throughout, so decisions are made in good time and the programme stays on track. The result is a build that runs predictably, with no surprises and a finish you can rely on. #Construction #Northampton
Frequently asked questions
How long does a single-storey extension take to build?
A straightforward single-storey extension typically takes around three to six months to build once work starts on site, according to the Federation of Master Builders. The exact timing depends on size, specification and ground conditions.
How long does a double-storey extension take?
Double-storey extensions usually take longer than single-storey ones, often four to six months or more, because there is an extra floor of structure, scaffolding and internal fit-out to complete.
How long does planning permission take?
In England, the local planning authority has a statutory eight-week period to decide most householder applications, as set out in the government guidance on determining a planning application. This happens before building work begins, so factor it into your overall timeline.
Can the build be done faster?
Finalising your design, confirming the specification and ordering long-lead items before work starts are the most effective ways to keep a build moving. Avoiding mid-project changes also prevents delays.
Do I need to move out during the work?
Most homeowners stay in the property throughout. Expect the noisiest, dustiest period during groundworks and any knocking-through, and plan a temporary kitchen if the main one is affected.
Ready to plan your extension?
If you are thinking about an extension in Northampton or the surrounding area, the best next step is a conversation about your site and your plans. Contact AAS Group for a realistic timeline tailored to your project, and we will talk you through what to expect from first dig to final handover.
