How long does an extension or loft conversion take?

Most of the calendar is not building. For a single-storey rear extension on a two-storey mid-terrace house, the on-site part is 8 to 16 weeks, and the whole job from first sketch to the completion certificate is 23 to 43 weeks. Design and the planning step come first, then the structural design and building regulations drawings, then the build, then sign-off.

The phases are shown in the table. The party wall notices run alongside the structural design and tender rather than after them, so they are left out of the total. The planning step shown is the one for the route this house is likely to take, which the engine labels "larger home extension: prior approval needed".

A loft conversion spends less time on site than a rear extension because the walls and roof are already there. For a loft conversion with a rear dormer on a two-storey mid-terrace house, the build is 6 to 10 weeks and the whole job is 23 to 39 weeks.

The design, structural and approval steps take about as long as for an extension, so a loft is quicker overall only because the building is shorter. A simple rooflight loft is the quickest kind and a mansard, which rebuilds the back of the roof, the slowest. The planning step shown is the one for this house, which the engine labels "likely permitted development (no planning application)".

The planning step is the part of the timeline that changes most with the house. For the typical rear extension it is prior approval, 6 to 8 weeks; for the typical loft it is lawful development certificate, 8 to 10 weeks.

If the work is normally permitted development, subject to the limits checked in the report, a lawful development certificate is optional but advised. The council has 8 weeks to decide once the application is validated, so allow 8 to 10 weeks. You can start work without it, but you then have no written confirmation if the question comes up later.

Where the work needs prior approval, such as a larger single-storey rear extension, the council consults neighbours for 21 days and must decide within 42 days, or approval is deemed. Allow 6 to 8 weeks. Where a full planning application is needed, allow 9 to 14 weeks: one to two weeks to validate, 21 days of consultation and an 8-week statutory period that often stretches. In a conservation area it can run longer, so check with the council.

The planning step also sets when other things can start. The structural design and tender can overlap it, but nobody should commit to a start date, or order steel, until the decision is in.

The calendar above starts when the design starts and assumes the build can begin when the drawings and approvals are ready. In practice a good contractor is often booked months ahead, so the tender phase is also when you reserve a start date. Three like-for-like quotes take a few weeks to collect, and the best-regarded firms may quote for a start further out than you would like.

Materials have their own lead times. Steel beams are cut to the engineer's calculations and are usually ordered once the design is fixed. Large glazing, such as bifold or sliding doors, roof lanterns and frameless glass, is made to measure and can take many weeks, so it should be ordered as soon as the opening sizes are fixed. A dormer's windows and a staircase made to measure for a tight landing are the same. A late order for one item can hold up a whole site.

The cheapest way to save time is to settle the specification before the build starts. Choosing the glazing, the kitchen and the finishes early lets orders go in while the groundworks are under way.

For a loft conversion, usually yes. The work is above your heads, the scaffold goes up outside, and the main disruption is noise, dust and a stair opening cut into the landing. There are short spells without water or heating when the plumbing is connected, and the days when the roof is opened are the ones to plan for. Many families stay put for the whole of the 6 to 10 weeks.

For a rear extension, usually yes, though less comfortably. The back wall comes out, so the kitchen is out of action for weeks, often for most of the 8 to 16 weeks on site, and the garden is a building site. Households often set up a temporary kitchen elsewhere in the house, with a microwave and a kettle, and wash up in the bathroom. Dust and noise are constant. A double-storey extension, a wraparound or work that opens the whole ground floor is more disruptive, and some households choose to move out for part of it.

Living elsewhere is not in the totals, so if you plan it, budget for it separately.

The ground. Foundations dug into clay, a tree root, a drain that is not where the plan says, or a public sewer close to the new walls can all stop work while an engineer or the water company is consulted. The ground is the hardest thing to know before it is open.

The party wall. A neighbour who does not consent in writing starts a formal process that takes weeks, and longer if surveyors are appointed. The tables allow 4 to 10 weeks and it runs alongside the design, so serve the notices early.

Discoveries. Old wiring, rotten joists, a chimney breast that is not what it seemed, or asbestos in an old roof can add time as well as cost. This is what the contingency is for.

Changes. Moving a wall, a window or a bathroom once the build has started means new drawings, new orders and sometimes a new building control submission. Changes after the design is fixed are the delay you control most.

Weather and the council. A wet winter slows groundworks and roof work, and a planning decision or a building control inspection that arrives late holds up the next stage.

How long does planning permission take for an extension?

It depends on the route. A lawful development certificate takes 8 weeks once validated, prior approval is decided within 42 days or deemed approved, and a full planning application has an 8-week statutory period that often stretches. Including validation, allow about 6 to 8 weeks for the planning step of the typical rear extension. A conservation area or an Article 4 direction can change the route, so check with the council.

Is a loft conversion quicker than a rear extension?

On site, yes. A typical rear dormer takes 6 to 10 weeks to build against 8 to 16 weeks for a typical single-storey rear extension. Overall the gap is smaller, 23 to 39 weeks against 23 to 43 weeks, because design, approvals and the drawings take a similar time for both.

Why is the total longer than the weeks on site?

Because the on-site weeks cover only the building. Before that come the design, the planning step, the structural design and building regulations drawings, and getting quotes; afterwards come snagging, building control's final inspection and the completion certificate. For the typical rear extension that is 8 to 16 weeks on site inside a 23 to 43 weeks total.