Modular and prefabricated extensions are sold on speed, and the speed is real but it is not where most people think. The groundworks, the drains, the connection into the existing house and the consents all happen exactly as they would on a traditional build. What is compressed is the weeks of trades on site, which is a genuine benefit and a partial one.
Prefab extensions: what is actually off site, and what is not
The frame, the insulation, often the windows and sometimes the internal finishes are made in a factory and craned or carried in. The foundation, the slab, the drainage, the opening cut into the existing wall, the making good, the electrics and the heating connection are site work either way. A supplier quoting a headline figure for the module alone is not quoting the project, and the gap between the two is where modular jobs disappoint.
Where the real advantages are
Factory conditions produce more consistent insulation and airtightness than a wet trade in February, and a shorter time with the house open to the weather. For a household living in the property that is worth a great deal. Quality control is repeatable, and the design is fixed early, which means fewer decisions made badly on site.
Where the constraints bite on ready made extensions
Access is the first one. A module has to reach the back of the house, and on a terrace with no side passage it usually cannot, which rules the method out before anything else is discussed. Craneage needs space, sometimes a road closure. And late changes are expensive in a way they are not on a masonry build, because the thing you want to change has already been made.
Questions for a modular house extension UK supplier before ordering
What exactly is in the price, and which of the site works are excluded. Who takes responsibility for the interface between the module and the existing building, which is where the leaks and the cold bridges happen. What warranty exists and who honours it if the supplier stops trading, since a deposit on a factory order is money at risk in a way a stage payment for work in progress is not. And whether the design will satisfy building control before anything is manufactured.
Questions people ask about modular house extension
Are ready built extensions cheaper?
Not usually, once the groundworks, drainage, craneage and connection into the house are included. It is generally faster on site and more predictable in quality.
Does it still need planning permission and building control?
Yes, on exactly the same basis as a traditional build. The method of construction does not change either regime.
What happens if the module does not fit?
This is why the survey and the setting out matter more than on a wet build. Insist on a measured survey before manufacture, in writing.
Is a deposit on a factory order safe?
Treat it as money at risk and ask what protection exists. Payment against evidenced manufacture beats a large payment on order.