How long does a home extension take? It is the question we are asked more than any other, and the honest answer is that time on site is only half the story. For a straightforward single-storey rear extension in Norfolk, allow roughly twelve to sixteen weeks from the first spade in the ground to a finished, decorated room. A two-storey extension typically runs to sixteen to twenty-four weeks. What catches most homeowners out is everything that has to happen before that: design, approvals and material lead times can comfortably add three to six months, and none of it can be compressed once a start date is booked. Here is what a realistic programme actually looks like, stage by stage.

Before the build: design, approvals and lead times

This is the phase people underestimate, and it is the one that decides whether your build starts on time. Running these items in parallel rather than one after another is what keeps the overall project short:

• Survey and drawings (2 to 4 weeks): a measured survey of the existing house, then plans and elevations for the proposed work.
• Planning permission (8 weeks or more): a householder application has a statutory eight-week determination period, and that clock only starts once the application is validated. If the work is permitted development, a Lawful Development Certificate is quicker and still worth having on file for when you sell.
• Structural design (2 to 3 weeks): beam and foundation calculations from a structural engineer, needed before Building Control can sign off the design.
• Building Regulations (allow 5 weeks): appoint a Building Control body early. This is a separate approval from planning permission, not an alternative to it.
• Party Wall notices (1 to 2 months' notice): where you are building at the boundary or excavating near a neighbouring structure. Neighbours have 14 days to respond, and a dissent means appointing surveyors, so serve early.
• Long-lead materials (4 to 12 weeks): steel beams, bi-fold or sliding doors, roof lanterns, bespoke joinery and matching facing bricks are the usual culprits. Ordering these late is the single most common cause of a stalled site.

Weeks 1 to 3: setting out, groundworks and foundations

Once work starts, the site is set up, protection and access are established, and the extension is set out from the drawings. The digger then goes in for foundation trenches and any drainage diversions. Building Control inspects the open trenches before concrete is poured, and existing drain runs are picked up and rerouted with new inspection chambers where needed. Foundations are poured and left to gain strength, then blockwork is brought up to damp-proof course level and the floor slab or beam and block floor is laid. Ground conditions drive this stage more than anything else, and much of Norfolk sits on variable ground where a trial pit before design work is money well spent. Our groundworks team handles this phase in-house, which avoids the handover delays that come with subcontracting it out.

Weeks 3 to 6: shell and weathertight

The superstructure goes up quickly compared with the groundworks. Cavity walls are built with insulation as they rise, the wall plate is fixed, steel beams are craned in and padstones bedded, and the roof structure follows. Felt, battens and tiles go on, flat roof coverings are laid, and windows and external doors are fitted. Reaching weathertight is the milestone that matters most in the whole programme, because from that point onwards the weather stops dictating progress and the internal trades can work regardless of what July or November throws at the site.

Weeks 6 to 10: structural opening, first fix and plastering

With the shell closed in, the opening between the existing house and the new space is formed and the supporting steel is loaded. This is the disruptive week, and it is worth planning around. Electricians and plumbers then carry out first fix, insulation and underfloor heating are installed, and the floor is screeded. Screed drying is a genuine programme constraint rather than something a builder can hurry: a traditional sand and cement screed needs roughly one day per millimetre of thickness for the first 50mm. Plasterboard and skim follow, and the plaster itself needs several days to dry before decorating can start.

Weeks 10 to 14: second fix, finishes and snagging

Second fix covers sockets, switches, lighting, radiators, sanitaryware and internal doors. Kitchens are fitted, floors laid, tiling completed and the room decorated. Externally, the scaffold comes down, brickwork is pointed and cleaned, guttering is connected and any patio, path or landscaping is finished. Building Control carries out the final inspection and issues the completion certificate, which is the document you will need when you sell. A snagging list is then worked through, and it is normal for that to take a further week or two.

What actually pushes a programme back

Genuine delays are rarely mysterious. In our experience they come from a short and predictable list:

• Ground surprises: tree roots, made ground, old foundations or undocumented drain runs that force a redesign of the foundations.
• Late material orders: a twelve-week lead time on glazing will stop a build that is otherwise ready to close in.
• Mid-build changes: moving a door or upgrading a specification once work is underway has a knock-on effect on every trade that follows.
• Party wall dissent: appointing surveyors and agreeing an Award can add weeks if notices were served late.
• Utility diversions: moving a gas main, water supply or electricity cable is measured in weeks by the utility provider, not days.
• Undecided finishes: tiles, kitchen units and sanitaryware need to be chosen well before the trade that fits them arrives.

How the timeline changes for other projects

A two-storey extension shares the same foundations and roof as a single-storey one, so the extra floor adds far less time than building twice. Expect sixteen to twenty-four weeks on site. A loft conversion is usually six to ten weeks because there is no groundworks phase, though staircase design and fire safety requirements add complexity. A wrap-around extension sits at the longer end because of the greater roof and drainage work involved. You can see the range of work we take on across Norfolk and Suffolk on our extensions page and in our case studies.

Why a late-summer start makes sense

Booking a start for August or September puts the groundworks and shell into settled, drier weather, which is exactly when you want them. Get weathertight before the clocks change and the whole of the internal programme, which is the longer half of the build, carries on through the autumn and winter unaffected. Leave it until October and the same foundations are being dug in wet ground, with shorter days and a real risk of frost delaying concrete and render.

If you are planning an extension in Wymondham, Norwich or anywhere across Norfolk and Suffolk, the most useful thing you can do is talk to a builder before the drawings are finalised, so the programme, the approvals and the material lead times are all set out realistically from the start. Get in touch with the Young's Construction team with your address, any sketches or plans you have, and the date you would ideally like to be finished by, and we will work backwards from there and tell you honestly what is achievable.