The honest answer is that it depends on scope — but working from realistic ranges is far more useful than a vague “it varies.” Most delays don’t come from the build itself; they come from what happens before it starts and from decisions that weren’t made early enough. Here’s what to expect at each stage, and where time most commonly gets lost.

Key takeaways

• A straightforward bathroom renovation takes 2–4 weeks on site; a full gut-and-refit can take 6–8 weeks
• A kitchen with structural work typically runs 4–8+ weeks; the fit-out phase alone can be 2–3 weeks
• An extension is usually 3–6+ months on site, but the full project from first conversation to completion is typically 7–12 months
• Design, planning, and preparation often take longer than the build itself
• Most delays trace back to late decisions, design changes, or hidden issues uncovered once work starts

Bathroom: 2–4 weeks on site, longer with complications

A straightforward bathroom renovation — new suite, new tiling, replumbing in the same positions — typically takes 2–4 weeks on site. That’s stripping out the existing bathroom, first fix plumbing and electrics, plastering, tiling, and fitting out.

What stretches that out:

Layout changes. Moving the toilet to a new position means new drainage runs, often including external groundwork to lay a new soil pipe. Add at least two weeks. Keep the toilet where it is if you can.

Structural surprises. Rotten floorboards, failing joists, damp behind old tiles, and leaking pipes are common discoveries once the bathroom is stripped. You won’t know until the work starts, which is why a contingency budget matters. Each of these adds time — sometimes days, sometimes weeks depending on severity.

Material lead times. Bespoke or special-order tiles, non-standard sanitaryware, and custom-fit shower enclosures can add weeks to a project if they’re not ordered before work begins. Order everything before the start date, not after.

Converting a room into a new bathroom. Installing a bathroom where there hasn’t been one before involves new soil pipe connections, usually building regulations approval, and potentially foundation work. Add several weeks and factor in the approval process.

The planning phase — choosing the layout, selecting every product, confirming lead times — typically takes 2–4 weeks on its own. The fastest projects are the ones where every decision was made before the builder arrived.

Kitchen: 4–8+ weeks depending on structural work

A kitchen renovation without any structural changes — like-for-like replacement of units, worktops, and appliances in an existing layout — can be done in 2–3 weeks. Most kitchen projects involve more than that.

Once you add structural work — knocking through to another room, removing a wall, adding a roof lantern or bifold doors — the timeline extends significantly. A kitchen-diner extension with structural steelwork, a new roof, and a full fit-out typically runs 4–8 weeks on site as a minimum, and complex projects can run longer.

The fit-out phase alone adds time that people often underestimate. A kitchen installation — units, worktops, splashback, integrated appliances, plumbing connections — is typically 1–2 weeks for a straightforward layout, more for bespoke cabinetry. Worktops that require templating (stone, solid wood) add a further week between template and delivery.

Structural work has a fixed sequence: steel goes in, structure is made good, then the trades follow. If the steel is delayed, everything behind it slips. Getting structural elements ordered early — and confirming lead times with your supplier before work starts — is one of the more effective ways to protect the programme.

Extension: 3–6+ months on site, 7–12 months total

The build programme for a typical single-storey rear extension runs 3–4 months on site. A double-storey extension is usually 4–6 months. Larger or more complex projects — wraparound extensions, significant structural reconfigurations, listed buildings — can run well beyond that.

But site time is only part of the picture. The full journey from first conversation to completion typically runs 7–12 months, sometimes longer. Here’s where the time goes before a brick is laid:

Design and surveys. An architect needs to produce planning drawings, which requires a measured survey of the existing property. Allow 4–6 weeks for this stage.

Planning permission. Cornwall Council’s statutory determination period is 8 weeks for a householder application. In practice, allow 10–12 weeks to account for information requests or minor delays. If the application needs revisions, add more. Projects within permitted development skip this stage, but it’s worth confirming your permitted development position early rather than assuming.

Building regulations. A separate process from planning, running alongside or after it. Building regs approval is needed before work starts and involves a building control officer checking plans and inspecting work at defined stages.

Tender and procurement. Once you have approved drawings, getting quotes from builders, comparing them properly, and confirming your contractor takes time — typically 4–6 weeks if done carefully.

Pre-start preparation. Materials with long lead times — structural steel, roof lanterns, bifold or sliding doors, bespoke glazing — need to be ordered before work starts, not once the groundworks are underway.

Take those stages together and a project that feels like “we’ll start in the spring” in January can realistically mean breaking ground in late summer if the first conversations don’t happen early enough.

Where delays actually come from

Most delays on building projects aren’t caused by slow trades or bad weather. They come from decisions that weren’t made in time, or information that wasn’t available when it was needed.

Design changes mid-build. Changing the layout, specification, or finishes once work has started almost always costs time as well as money. Trades may need to return to undo and redo work; materials already ordered may not be suitable. Every change has a programme cost. Make decisions before the project starts, not during it.

Late decisions. A builder on site waiting for a tile selection, a worktop template, or a confirmation of which door handle to use is a builder whose time is being wasted. Programme the decision-making as carefully as the build itself, and make choices earlier than feels necessary.

Hidden issues. Old properties in Cornwall — granite-built, timber-framed, or extended in previous decades — regularly throw up surprises: buried drainage runs, unexpected structural elements, inadequate existing foundations, damp. These can’t always be foreseen, which is exactly why contingency budgets and programme contingency exist.

Weather. Groundworks, brickwork, and roofing all have weather dependencies. Cornwall’s Atlantic climate means significant rainfall throughout the year. A builder can work around some weather, but not all — particularly during foundation and structural stages.

Material lead times. Supply chains have been less predictable in recent years. Structural steel, specialist windows, and certain roofing products can have lead times of 8–12 weeks. Ordering after planning approval — rather than on approval — adds that time to the back end of the project.

What good preparation actually looks like

The projects that run smoothly are nearly always the ones where the groundwork was done properly before the site started. That means:

Appointing a designer and starting the planning process well before you need work to begin. If you want to be on site in March, the first design conversation needs to happen in the summer or autumn before.

Making every product decision — tiles, sanitaryware, kitchen, windows, doors, flooring, lighting — before the start date. Not during it.

Confirming lead times on every long-lead item and ordering them in advance. Your builder or contractor should be tracking this as part of the programme.

Having a contingency — both budget and time — built into your expectations from the start. Ten to fifteen per cent on the budget and a couple of weeks of float in the programme is standard practice, not pessimism.

Frequently asked questions

How long does a bathroom renovation take from start to finish?

A straightforward bathroom renovation — new suite, tiling, and replumbing in the same positions — takes 2–4 weeks on site. A more complex renovation involving layout changes, structural work, or a conversion from a non-bathroom room will typically take 6–8 weeks or more. Factor in 2–4 weeks of planning and product selection before work starts.

How long does an extension take from planning to completion?

For a typical single-storey extension, the full journey from first conversation to completion is usually 7–12 months. Design and surveys take 4–6 weeks, planning permission takes 8–12 weeks, and the build itself runs 3–4 months. Projects requiring more complex planning or structural work will take longer.

What causes the most delays on building projects?

Late decisions and design changes once work has started are the most common causes of delay, ahead of weather or supply issues. Decisions about tiles, worktops, doors, windows, and finishes need to be made — and products ordered — before work starts, not during the build.

Can I stay in my house during an extension or renovation?

Usually yes, though it depends on what work is being done. Most bathroom and kitchen renovations allow you to remain in the house, though you’ll have limited or no access to the room being worked on for the duration. Extensions are generally more disruptive, particularly during groundworks and when the existing structure is opened up. Discuss this with your contractor at the planning stage.

Does Cornwall Council take longer than 8 weeks to decide a planning application?

The statutory determination period is 8 weeks for a householder application. In practice, it’s sensible to allow 10–12 weeks. Cornwall Council may issue requests for further information, which pause the clock and restart it. Applications in conservation areas or for listed buildings are typically more involved.