How Long Does an Extension Really Take?

Most homeowners guess six to twelve months for an extension, start to finish. Some projects genuinely do finish in five. Others take well over a year. The gap between those numbers is not random. It comes down to a handful of predictable choke points, planning being decided quickly or slowly, whether building control inspections get booked on time, whether a resubmission is needed. Once you can see where a timeline actually slips, you can plan around it instead of just hoping. This is the honest version, phase by phase, with the running total shown at every step so nothing gets rounded away.

In this article

• The Honest Timeline: What Each Phase Takes
• Why Planning Delays Are the Biggest Variable
• Building Control and Inspections: The Second Bottleneck
• Construction Phase: What Determines Speed
• The Real Timeline, Added Up Properly
• How to Stay On Track (And Save Time Where You Can)
• FAQ

The Honest Timeline: What Each Phase Takes

Here is a realistic phase breakdown for a typical rear or side extension across London boroughs (West London: Ealing, Harrow, Hillingdon, Hounslow; North London: Barnet, Brent, and others). Each phase below adds to a running total, so you can see exactly where the months actually go.

Phase	Typical duration	What’s happening
1. Initial design and concept	2 to 3 weeks	First meeting, spatial concept, initial sketches
2. Detailed design and planning drawings	2 to 4 weeks	Measured survey, drawings to submission standard, structural thinking behind the scenes
3. Pre-application consultation	4 to 6 weeks	Informal council feedback before formal submission, typically £200 to £400
4. Planning application and decision	8 weeks	Statutory determination period, longer if it goes to committee
5. Building control approval	1 to 3 weeks	Structural calculations, building regulations drawings, plus safety, insulation, drainage, and fire safety review
6. Pre-construction preparation	2 to 4 weeks	Contractor confirmed, materials ordered, site set up

Add the first three phases together and you get 8 to 13 weeks, roughly 2 to 3 months, before a formal planning application even goes in. That number surprises most people, because nothing about it looks like delay. It is three separate, legitimate stages, each doing real work.

Add phase four and the running total reaches 16 to 21 weeks, about 3.5 to 5 months, from your first design conversation to a planning decision. Add phases five and six and you are at 19 to 28 weeks, roughly 4.5 to 6.5 months, before a contractor breaks ground. Construction has not started yet. That is the part most timelines quietly leave out.

If your property is listed or in a conservation area, add 1 to 2 weeks to phase two for the extra material and proportion detail heritage officers expect, and see the conservation area note in the next section for what it adds to phase four.

Why Planning Delays Are the Biggest Variable

Planning is where most extensions lose weeks unexpectedly, and it is the single biggest reason two identical looking projects finish months apart.

Approved first time: 8 weeks, the statutory determination period, and you move straight to building control.

Approved after an information request: add 2 to 4 weeks. Planning asks for clarifications, amended drawings, or an additional survey (ecology, structural engineer opinion, and similar), you resubmit, and they take another 2 to 4 weeks to reconsider.

Refused, then resubmitted: this is the expensive one. A refusal arrives at the end of the 8 week determination period, and a redesigned resubmission restarts that same clock. Budget 16 weeks total for planning if a refusal happens along the way, double a clean approval, plus whatever time your redesign itself takes. The lesson is not “avoid refusal by luck.” It is “spend the £200 to £400 on pre-application advice in phase three,” because that is the step most likely to catch a refusal-worthy issue before it costs you another 8 weeks.

Conservation area or listed property: add 2 to 4 weeks to the planning decision phase for heritage consultation. West London in particular (Ealing, Harrow) has a notable concentration of conservation areas, so check this before you assume a standard 8 week decision.

Neighbour objections: if neighbours object during the consultation period, the application may be pulled from delegated decision into a committee hearing. Committee meetings typically sit 4 to 8 weeks apart, so a hearing can turn an 8 week decision into 12 to 16 weeks even without a refusal.

Building Control and Inspections: The Second Bottleneck

Building control itself is fast, usually 1 to 3 weeks for the initial approval, once your structural calculations and building regulations drawings are submitted alongside the application. Where projects actually stall is during construction, waiting for inspections that were not booked early enough.

Expect 4 to 5 site inspections over the course of a build: foundations, before the concrete pour; structural, before walls close up; electrical first fix, before walls are finished; and a final inspection before you move in. Each one needs booking 5 to 7 days ahead. If your contractor requests them late, or the inspector cannot attend when needed, you lose roughly a week per missed slot, and four missed slots is a month of otherwise avoidable delay.

The fix is procedural, not technical: your contractor should request each inspection 10 days before it is needed, and you should be the one checking that the request actually went in, not assuming it did.

Construction Phase: What Determines Speed

Construction duration depends almost entirely on scale and structural complexity, not on how fast a crew works.

Project type	Construction time	Why
Rear extension, single storey, around 20 sqm	12 to 16 weeks	Demolition 1 to 2 weeks, foundations 2 to 3 weeks, structure 3 to 4 weeks, finishes 4 to 6 weeks, snagging 1 to 2 weeks
Side return, typically two storey, 30 to 40 sqm	16 to 24 weeks	More structural work, and the tie in to the existing building is more intricate
Loft conversion, 20 to 30 sqm	12 to 18 weeks	No new foundations needed, but dormer and roof work are weather dependent
Two storey extension, rear or side, 40 to 60 sqm	24 to 36 weeks	Structural complexity plus multiple building control inspections and more coordinated trades

Adding more people to a construction phase rarely speeds it up. Foundations, structural frame, and first fix trades each depend on the stage before them finishing, so the real levers are booking building control early and having materials on site before they’re needed, not headcount.

The Real Timeline, Added Up Properly

Here is what happens when the design and planning phase (4.5 to 6.5 months before groundbreak, from the table above) is actually added to construction, rather than quoted as a separate, disconnected figure.

Project type	Design and planning to groundbreak	Construction	Total, first conversation to completion
Rear extension, single storey	4.5 to 6.5 months	3 to 4 months	About 7 to 10 months
Side return, two storey	4.5 to 6.5 months	4 to 6 months	About 8 to 12 months
Loft conversion	4.5 to 6.5 months	3 to 4.5 months	About 7 to 10.5 months
Two storey extension, rear or side	4.5 to 6.5 months	6 to 9 months	About 10 to 15 months

That is longer than the five to eight months you’ll see quoted elsewhere, and it’s longer on purpose: it’s the design and planning phase and the construction phase added together, rather than either one quoted alone. Five to eight months is roughly what design and planning takes by itself, on the fast end. It was never the whole project. If a builder or a website gives you a single number for “how long will my extension take,” ask whether it includes the months before the first spade goes in. That is usually where the gap comes from.

None of this means slower is safer. A clean pre-application conversation, a straightforward design, and a contractor who books inspections on time can land you near the fast end of every range above. The point of adding it up properly is so your fast end and your realistic end are both numbers you can actually plan around, not a guess.

How to Stay On Track (And Save Time Where You Can)

Do a pre-application consultation. This single step is the biggest lever in the entire timeline, because it is the only phase that specifically exists to catch a refusal before it costs you another 8 weeks.

Order long lead materials early. Roof trusses, windows, and specialist brickwork can take 6 to 12 weeks to manufacture. Your contractor should place these orders before building control approval comes through, not after, so they’re sitting on site rather than holding up the structural phase.

Confirm building control inspection dates before work starts. Do not assume an inspector will be available the day you need them. Book ahead, and check that your contractor actually made the booking.

Plan around weather. Foundation concrete needs dry conditions, and roof work is weather dependent. June through September is the safer window. November through February will slip if there’s heavy rain or frost, sometimes by several weeks.

Use the waiting phases productively. While planning is being decided, that is dead time only if you let it be. Finalise your contractor, confirm your budget, and settle on finishes, so nothing new is holding up phase five once the decision lands.

Frequently Asked Questions

What’s the fastest extension timeline you’ve seen?

Around three months from first design conversation to completion, but that required a pre-application conversation completed before design even started, a simple rear extension with no structural complexity, a contractor ready to start immediately after building control, and good weather throughout. Treat it as the exception, not the plan. Four and a half months for design and planning alone is more realistic even on a fast project.

Can I run planning and detailed design in parallel to save time?

Technically yes, but it’s risky. Start construction drawings before you’ve had planning feedback and you may end up redesigning around a condition or objection you didn’t see coming. Getting pre-application feedback before detailed design costs a few weeks up front and usually saves more than that in avoided rework.

Why does building control take so long after planning is approved?

It usually doesn’t. Building control approval itself is typically 1 to 3 weeks. The gap people actually feel is phase six, pre-construction preparation, confirming a contractor, ordering materials, organising site access, which is genuine lead time, not a building control delay.

If my application gets refused, can I resubmit immediately?

You can, but it’s rarely wise to. Take the time to understand exactly why it was refused, amend the design to address that specific reason, and resubmit once you’ve genuinely fixed it, typically 2 to 4 weeks later. Resubmitting the same design quickly tends to produce the same refusal.

What does a conservation area actually add to the timeline?

Around 2 to 4 weeks to the planning decision phase for heritage consultation, plus roughly 1 to 2 weeks to the design phase for the additional material and proportion detail heritage officers expect. The construction phase itself is unaffected.

Can the construction phase be sped up?

Adding more workers rarely helps, because foundations, structural frame, and first fix trades each depend on the stage before finishing, so more hands mostly means more coordination overhead. Weather and material delivery delays are the two genuine variables. Your real lever is booking building control inspections early and keeping materials on site ahead of when they’re needed.

What happens if my contractor falls behind schedule?

If the delay is outside their control, weather, a material shortage, a building control finding, you’re typically entitled to an extension of time under most standard contracts. If the delay is down to them, you can pursue the matter through the contract or bring in another contractor to finish. This is exactly why a clear written contract matters before work starts, not after a delay happens.

Is winter a bad time to start construction?

Generally yes. Concrete needs dry conditions to cure properly, and roof work is riskier in heavy rain. Extensions starting construction in November or December often slip 4 to 8 weeks against the estimate. June through September gives you the most reliable weather window.

Phase one and phase two are similar lengths, so what’s actually different between them?

Phase one, 2 to 3 weeks, is a conversation and a concept sketch. Phase two, 2 to 4 weeks, is where that concept becomes a measured, dimensioned, submission standard drawing set, structural thinking, building regulation checks, and site constraint analysis all happening even though none of it is visible in a single sketch. They sit close together in length because both are genuinely necessary steps, not because either one is padded. Skipping straight from a concept sketch to submission drawings is usually where amendments creep in later.