Oak frame extension kits: what you get and what you do not

Search for oak frame kits and you will find prices that look like a bargain beside the cost of a finished extension. They are not comparing the same thing. A kit is one part of a building, and whether it saves money depends on who does the rest.

What a kit contains

A typical kit is the structural frame and nothing else:

• posts, wall plates, tie beams, trusses and braces in green oak, with every joint cut;
• oak pegs, usually a few spare;
• each timber marked with a number that matches a set of assembly drawings;
• delivery to the kerb, often on a lorry with its own crane arm.

Some suppliers add softwood common rafters, and some will quote for glazing or for a team to put the frame up. Each of those is an extra, and you should ask for it to be priced separately so you can compare like with like.

What is not in the box

Everything that turns a frame into a room:

• drawings for planning and Building Regulations, unless the supplier offers them;
• foundations, the floor slab and the plinth the frame stands on;
• lifting and assembly;
• roof insulation, battens, tiles or slates, and leadwork;
• wall insulation and external cladding or render;
• glass and the system that fixes it to the oak;
• the opening into the house and any beam over it;
• electrics, heating, plaster, flooring and decoration.

On a finished oak frame extension, the frame commonly accounts for a quarter to a third of the cost. A kit price tells you about that share only.

Standard sizes or made to measure

Kits come two ways. Catalogue kits are fixed designs in set widths and depths. They are cheaper because the drawings and cutting lists already exist, and they work well for a porch or a simple lean-to on a house with ordinary proportions.

Made-to-measure kits are drawn for your house and then supplied frame-only. This is how most extension frames are bought when a general builder is running the job. It costs more than a catalogue kit and less than a full design-and-build service.

An extension has to meet the house at a particular floor level, miss particular windows and match a particular roof. A catalogue design rarely does all three without changes.

The rules still apply

Buying a kit changes who does the paperwork. It does not remove any of it.

Planning. The frame needs the same permission as any other extension of its size and position.

Building Regulations. Building control will want structural calculations for the frame, the foundations and the roof. Ask the kit supplier whether an engineer’s calculations for your design are included. For a catalogue kit they often are; for anything altered they may not be.

Insulation. The oak provides none. Whoever designs the walls and roof round the frame has to show that they meet the current standards.

Health and safety. On a domestic job, the legal duties for managing safety pass to your contractor. If you employ separate trades yourself with nobody in overall charge, more of that responsibility comes back to you.

Putting it up

Green oak is heavy, roughly a tonne per cubic metre. A 200 mm square post 3 m long weighs about 125 kg, and a tie beam can be twice that. Frames are raised with a crane or a telehandler by people who have done it before, and a small extension takes two to four days.

A good carpenter who has never raised an oak frame can manage a porch. A two-truss garden room is a different matter. Many kit suppliers will send one experienced framer to lead your builder’s team for the raising, which is money well spent.

Before the frame arrives, the plinth must be level and square to within a few millimetres. A frame cut to the drawing will not fit a base that is 20 mm out, and oak cannot be packed and bodged like softwood studwork.

Where kit projects go wrong

The trouble is nearly always at the joins between trades.

• The builder sets the plinth to the wrong height because the frame drawings arrived late.
• Glass is ordered from the drawings instead of measured from the raised frame, and does not fit.
• The roofer and the framer each assume the other is weathering the junction with the house.
• Nobody plans for shrinkage, and plaster is run tight to green oak that then pulls away from it.

With a single firm doing everything, those are its problems. With a kit, they are yours.

Is a kit right for you?

A kit makes sense if you have a builder you trust who has worked with green oak, a designer handling the approvals, and the time to coordinate deliveries. It also makes sense for small, simple structures.

It makes less sense if this is your first building project, if the house is listed, or if the design has a lot of glass. In those cases the saving is small beside the risk.

To compare the finished cost of different sizes, use the cost page. To get a price from a framer, whether frame-only or the whole build, fill in the form below.