The short answer
For a typical UK extension, the foundation stage usually takes around one to two weeks from starting the dig to being ready to build off the concrete, though a simple single-storey footing can be quicker and difficult ground can take longer. The sequence is: set out and excavate the trenches (often 1–3 days), have Building Control inspect the open trench, pour the concrete (often a single day), and then allow it to gain strength before loading it — bricklaying off a footing commonly begins after a few days, with concrete reaching most of its strength over about 28 days. The pace is set less by the digging than by inspections and curing: you cannot pour until the trench is passed, and you cannot fully load the foundation until the concrete has cured. Deep trenches, clay, trees, services or bad weather all extend the timetable.
People imagine foundations are slow because of the concrete; in practice the dig is quick and the waiting is for inspection and curing. Here is a realistic timeline.
Foundation timescale
- Typical extension stage~1–2 weeks
- ExcavationOften 1–3 days
- Inspection before pourBuilding Control
- PourOften 1 day
- Concrete strengthMost gained by ~28 days
The stages and their timing
- Setting out & excavation (1–3 days): mark the trenches and dig to the design depth. Hand-digging around services or in tight access takes longer than a machine dig.
- Trench inspection (same/next day): Building Control inspects the open trench. The pour cannot proceed until they accept the depth, width and bearing.
- Pour (often 1 day): concrete is placed, usually ready-mixed and pumped or barrowed in. Trench fill uses more concrete but is quick to place.
- Curing & building off (days to weeks): bricklaying off a footing typically starts after a few days; concrete continues gaining strength, reaching most of it by around 28 days.
| Stage | Indicative time | Note |
|---|---|---|
| Set out & excavate | 1–3 days | Longer in tight access |
| Building Control inspection | Same/next day | Cannot pour before |
| Concrete pour | ~1 day | Trench fill is quick to place |
| Initial cure before building | ~2–4 days | Before loading the footing |
Indicative timings for a typical extension; difficult ground extends them. Sources: LABC inspection guidance; NHBC Standards Chapter 4.
Why inspections and curing set the pace
Two fixed points control the timetable. First, the open-trench inspection: you must give Building Control notice and wait for acceptance before pouring, and if they require extra depth the dig restarts. Second, concrete curing: fresh concrete is weak and gains strength over time, so the foundation should not be heavily loaded too early. Builders manage this by starting lighter work soon and timing heavier loading later. Rushing either step is the usual cause of foundations having to be broken out and redone.
What causes delays
Foundations overrun for predictable reasons: deeper-than-expected ground needing more digging; clay and trees forcing deep trench fill or piling; unmapped services (water, gas, drains) found in the trench; standing water needing pumping; and weather — heavy rain floods trenches and freezing conditions delay pouring. A realistic programme allows contingency for these rather than assuming a clean run. On piled jobs, add time for the specialist rig and pile installation before the ground beams are cast.
How the foundation type changes the timeline
"Laying foundations" covers very different jobs, and the type you need can swing the timetable from a few days to a couple of weeks or more.
- Strip / trench fill (firm ground): the quickest route — dig, inspect, pour, and you are building off within days. A simple single-storey extension can complete the foundation stage in under a week if the weather and ground cooperate.
- Deep trench fill (clay near trees): the same sequence but more excavation, more concrete and a deeper, more carefully inspected trench, adding days to the dig and the pour.
- Raft: add time for the sub-base preparation, fixing reinforcement, and the engineer's and Building Control inspections of the steel before a larger pour.
- Piled with ground beams: the longest — mobilising a specialist rig, installing and recording the piles, then excavating, reinforcing and casting the ground beams. This can run to two weeks or more before you build off, plus lead time to get the rig to site.
Two practical levers keep the programme honest. First, sequence the trades: get the ground investigation, design and any drainage or build-over agreements sorted before the dig, so the foundation is not held up waiting for paperwork. Second, book inspections and concrete in advance: give Building Control proper notice for the trench (or steel) inspection, and order ready-mixed concrete only once the inspection is passed, so the lorry is not sitting idle or, worse, arriving before the trench is approved. Treating the foundation stage as a coordinated sequence rather than a single task is what turns the textbook "one to two weeks" into a timetable that actually holds.
Where the foundation stage sits in the whole build
It helps to see the foundation stage in the context of the wider extension, because its real length includes the work that happens before the first spade goes in. The dig itself is quick; the lead-in and the curing are what people underestimate.
- Before the dig: design, the ground investigation, any structural engineer's input, and — where a drain or public sewer is involved — a build-over agreement or diversion all need to be sorted first. Started late, these hold up the dig even though they are not groundworks themselves.
- The dig and pour: setting out, excavation, the open-trench inspection and the pour are typically a matter of days on a straightforward extension.
- Curing and building off: light bricklaying often starts within a few days, with the concrete reaching most of its strength by around 28 days — you do not wait the full month, but heavier loading is timed sensibly.
- Then the superstructure: once you are building off the foundation, the extension moves on to walls, floor, roof and fit-out, which dwarf the foundation stage in total time.
The foundation is therefore a small part of the overall programme but a critical-path one: almost nothing above ground can start until it is in and cured, so a delay here pushes everything back. Two habits keep it honest — get the design, ground investigation and any drainage permissions done before the dig, and give Building Control proper notice for the trench inspection so the pour is not waiting on a booking. Treated that way, the textbook "one to two weeks" for the foundation stage holds, and the rest of the build can follow without the early slip that so often sets an extension behind schedule.
Frequently asked questions
How long before you can build on new foundations?
Bricklaying off a footing often begins after a few days once the concrete has set enough to take initial load. Concrete continues gaining strength, reaching most of it by around 28 days, but you do not wait the full month to start building.
Can foundations be poured in winter?
Yes, with care. Freezing conditions delay or prevent pouring, and trenches can flood, so contractors may use admixtures, protection or simply wait for a suitable window. Winter generally adds time to the foundation stage.
Why is my foundation taking longer than quoted?
The usual reasons are deeper ground than assumed, services found in the trench, water needing pumping, or weather. Building Control requiring extra depth at the trench inspection is common and not a fault — it protects the structure.
Sources & further reading
- LABC — Building Control inspections
- NHBC — Standards Chapter 4 (foundations)
- Planning Portal — building an extension (Building Regulations)
Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.