What is in an extension kit, and what is not

Flat pack, self build and DIY extension kits are the same product under three names: the structural shell, made off site and delivered in pieces. What suppliers count as "the shell" varies more than the adverts suggest. This page sets out what you can expect, what to ask about and what is never included.

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In the kit

Walls: panels or frames, cut and numbered
Roof structure: joists, rafters or roof panels
Sole plates
Structural fixings
Calculations for the frame
Assembly drawings
Delivery to the kerb

Not in the kit

Foundations and the floor slab
The opening into the house
Drains and manholes
Electrics, heating and plumbing
Plaster, flooring and decoration
Planning and building control fees
Scaffolding, skips and plant hire

The grey area: ask about each of these

Between what is always in and what is never in sits a list that changes from one supplier to the next. Two quotes thousands of pounds apart can turn out to be the same price once these are lined up.

Item	How often it is included	What to ask
Insulation	In SIPs by definition. Often left out of open timber panels	The thickness and type needed to reach 0.18 W/m²K
Windows and doors	Sometimes, as an extra	Whether openings are sized for standard frames
Roof covering	Rarely. The deck is included, the membrane is not	What the deck is rated to carry, and the fall built into it
Roof lantern or rooflights	Sometimes	Whether the opening and its trimmers are in the frame price
Cladding and battens	Sometimes	Whether the fixings and cavity barriers are included
Internal lining	Rarely	Which vapour control layer the wall design relies on
Putting it up	Offered by most suppliers at extra cost	The day rate or fixed price, and what the crew needs on site

What is never in the box

The ground. Footings, the slab and any drains under it are built on site by you or a groundworker, to a drawing the kit supplier does not produce. The supplier will tell you the loads; a structural engineer or building control settles the footing.

The hole in the house. Joining the new room to the old means taking out part of the back wall and holding up what is above it with a steel beam. That beam needs its own calculation, temporary props and a builder who has done it before. It is the riskiest part of the job and no kit touches it.

Approvals. The supplier's calculations support your building regulations application but do not replace it. You or your designer apply, pay the fee and book the inspections. If the extension needs planning permission, that is yours to obtain as well.

Everything that makes it a room. Wiring, heating, plaster, floor finishes, a kitchen if there is one.

Nine questions that make quotes comparable

Is delivery in the price, and to where: kerb, driveway or back garden?
Does the lorry unload itself, or do I need people or a machine?
What is the wall thickness, outside face to inside face, once finished?
What U-value does that wall reach, and is the insulation in the price?
Do the calculations cover the whole frame, including fixing it down and bracing it?
How level and square does my base need to be?
How long from deposit to delivery, and how much notice do I get of the date?
What are the payment stages, and how is my deposit protected?
If a part is missing or wrong, how quickly is it replaced and who pays?

Put the answers side by side. The cheapest kit is quite often the one with the most left out.

The order things happen in

Drawings and approvals come first, because the kit is made from the approved drawings and cannot be changed cheaply afterwards. The base goes in while the kit is being made. The kit is delivered, usually on one lorry, and the shell goes up. Then the roof is covered and the windows fitted, which makes the room weathertight. Only after that is the wall of the house opened, so the house is never exposed. First fix, plaster, second fix and floors follow, and building control signs the work off at the end.

How much of the job the kit is

By cost, a bit under half. National guides put kits at £1,000 to £1,400 per m² of floor and finished timber frame or SIPs extensions at £2,200 to £3,000 per m². The kit price guide shows the split for your floor area.

Where our information comes from

Rules quoted are for England unless we say otherwise. Wales follows closely; Scotland and Northern Ireland have their own building standards.

What is in a kit: common questions

Are flat pack extension kits and self build extension kits different things?

No. Both names describe a structural shell made off site and delivered in pieces for assembly. "Self build" stresses who puts it up; "flat pack" stresses how it arrives. Suppliers use them interchangeably.

Do extension kits come with instructions?

They come with assembly drawings: a plan showing where each numbered panel goes, and details for the joints. They assume you can read a construction drawing. If you cannot, pay for the supplier's crew or a carpenter for the frame stage.

Is the kit supplier responsible if the extension fails building control?

Only for what they supplied. If the frame is not as calculated, that is theirs to put right. If the footing is too shallow or the insulation you bought is too thin, that is yours. This is why the split of who supplies what should be written into the order.

Can I buy a kit before I have approval?

You can, but do not. If planning or building control asks for a change, a kit already made to the old drawing may be scrap. Order once the drawings are approved.

Keep reading

Timber frame kitsStud panels, the usual first choice SIPs kitsInsulated panels, thinner walls Kit pricesKit price against finished price

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