The short answer
The quickest extension to build is a factory-finished module, which is typically on site for two to six weeks. Panel-built extensions take four to eight weeks, and brick and block takes ten to sixteen. Whatever the method, design, approvals and factory lead time add two to four months before anyone arrives.
On this page
Four methods, ranked by time on site
“Quick” can mean two things: how soon the room is ready, or how short a time you live with builders. The second is where build methods really differ.
| Method | Typical time on site | What happens at the house |
|---|---|---|
| Finished module | 2 to 6 weeks | Foundations, one crane day, connections |
| Panel-built | 4 to 8 weeks | Foundations, shell up in days, then fit-out |
| Kit, self-managed | 6 to 12 weeks | As panel-built, at the pace your trades allow |
| Brick and block | 10 to 16 weeks | Everything built by hand, outdoors |
These are typical figures for a single-storey room, from guides published in 2026. The kit range is our own estimate.
Why factory methods are quicker
Three reasons, and none of them is magic.
Work overlaps. With brick and block, nothing can start above ground until the foundations are finished. With a prefab extension, the walls and roof are being made in a factory during the same weeks the groundworker is digging.
Weather stops mattering. Bricklayers lose days to rain and frost, and wet blockwork must dry before plastering. A panel shell is roofed and sealed within a week of delivery, so the inside trades can start at once.
There is less to do. A closed panel arrives insulated and lined. A finished module arrives wired, plastered and sometimes floored. Each task done in the factory is a task nobody does in your garden.
The fastest of all: a module
A volumetric module is a complete room on a lorry. A crane sets it on its foundations in a morning, and the remaining work is joining it to the house: cutting through, connecting power, water and drains, and sealing the junction.
The catch is access. The crane needs firm ground within reach of the garden and a clear path overhead. Narrow streets, overhead cables and big trees can rule it out. Where they do, panels carried through by hand are the next quickest thing.
What no method can speed up
Every extension has a run-in before the first spade goes in.
Design. A month or two for a survey, drawings and an engineer’s calculations. Factory methods need every decision made early, because the panels are cut to the drawing.
Planning. If you need permission, the council has eight weeks to decide a householder application. A larger rear extension under the prior approval scheme takes up to 42 days. Staying inside permitted development avoids both.
Building regulations. A full plans application can take five weeks to be approved. Many firms start on the strength of a submitted application, but approved plans are safer.
Party wall notices. In England and Wales you must give neighbours one or two months’ notice of certain work near the boundary. Serve it as soon as the design is fixed.
Factory lead time. Three to six weeks for timber panels is typical, and longer in spring when order books fill. Modules take longer, since more is built into them.
Add these together and even a quick build extension is usually three to five months from first sketch to finished room. The gain over brick is that only a few of those weeks are spent with a building site outside the back door.
How to keep a fast build fast
- Fix the design, then stop. One change after the panels are ordered can cost weeks.
- Order windows and doors with the panels. Glazing has the longest lead time of any component.
- Have the base finished and tested for level before delivery day. Factory panels will not adjust to a slab that is out.
- Line up the electrician and plasterer for the week after the shell stands, not the week you remember.
- Serve party wall notices and apply for building regulations in parallel with the factory order.
- Choose finishes that are in stock.
Does quick cost more?
Not much either way. Panel-built extensions are typically £2,200 to £2,800 per m² and finished modules £2,100 to £3,300, against £1,800 to £3,000 for brick and block, all national ranges including VAT. Some of the difference comes back through shorter hire of scaffolding, skips and site toilets, and through fewer weeks of takeaways while the kitchen is out of action.
Whether that trade is worth it depends on how you live. A family with small children and one kitchen tends to value six fewer weeks of dust more than a retired couple who can go away.
Is the quality the same?
A factory-made extension has to pass the same building regulations as a hand-built one, with the same inspections. Factory conditions help with accuracy and dryness. The weak points are the junctions made on site: where the new room meets the old wall, and where the roof meets the house. Ask who is responsible for those details and how they are sealed.
If speed is your priority, tell us in the form below how you reach the back garden. That one answer usually decides which quick method is open to you.