Prefab houses: timber frame, panels and modules

Timber frame kits range from open panels, where the factory delivers wall frames and the insulation and boards are added on site, to closed panels that arrive insulated, lined and sometimes with windows fitted. Structural insulated panels (SIPs) are sandwich panels of insulation between boards and form walls and roofs together. Volumetric modular homes arrive as finished boxes, complete with kitchens and bathrooms, and are craned onto the foundations.

The more work done in the factory, the shorter the time on site and the earlier every decision has to be made. Some companies supply only the structural kit; others offer a package up to a weathertight shell, or a finished house through their own or partner builders. Be clear which one you are being quoted for.

Factory production needs final drawings. Once the design is frozen, moving a window, a wall or a socket may mean a new panel or a charge for the change. Planning permission, structural calculations and Building Regulations approval should be in place, or at least well advanced, before the design is frozen.

Manufacturers usually take a deposit to reserve a production slot. Ask what happens to that deposit if the manufacturer stops trading, whether it is protected, and at which stages further payments fall due. Paying for panels that are sitting in a factory rather than on your plot is a risk you should understand.

A factory frame is precise, so the foundations must be too. They are usually built by a groundworker working to the manufacturer's tolerances, and a slab or wall that is out of level or out of square causes delays and extra cost on the day of erection. Agree who checks the foundations before delivery and who pays if they are wrong.

Large panels and modules arrive on long lorries and are lifted by crane. Narrow lanes, low bridges, overhead cables, trees and parked cars can all block delivery. Road closures or crane positions may need permission from the council. A manufacturer should survey the access before confirming the price.

Prefab houses need the same planning permission and Building Regulations approval as any other new home. The manufacturer's design and certification help, but Building Control still inspects the foundations, drainage and the completed house. Fire stopping between panels and floors, and moisture control in timber walls, are details to ask about.

Mortgage lenders and warranty providers are used to timber frame, but some treat other factory systems as non-standard construction. Before committing, ask the manufacturer which structural warranties and lenders have accepted its homes, and check with your lender or broker. Self-build mortgages that release money in stages need stages that match the factory's payment plan.

• exact scope: kit only, weathertight shell, or finished house
• wall, floor and roof build-ups, and the thermal and airtightness targets
• design freeze date and the cost of later changes
• deposit, payment stages and how deposits are protected
• foundation tolerances, who builds them and who checks them
• delivery, crane, access survey and any road closures
• warranty provider, lender acceptance and the certificates you will receive

We show your project, without your personal details, to prefab manufacturers and builders who cover your area. One company can buy it and pays Offrano for it; you pay nothing. Before it sees your name and number, we email you its name, address and phone number.

No company has to take the request, and after 21 days without a buyer it is withdrawn. You are free to say no to the quote. Offrano checks only that buyers are registered businesses, so visit completed homes and speak to their owners before you sign.