SIPs vs timber frame vs modular garden rooms

The short answer

The wall system is one choice among many. The three terms describe different things, which is why people mix them up.

• Timber frame and SIPs describe how the walls are made.
• Modular describes how the room is built and delivered.

So a modular room can have a timber frame or SIPs walls. When you compare quotes, ask what the walls are made of, and where the room is built.

Timber frame

A timber frame is a structure of upright timbers. Insulation goes between them. Boards, membranes and cladding go on the outside. Boards and plasterboard go on the inside.

• Widely used, and familiar to builders.
• You can choose the type of insulation, since it is not fixed to the panel.
• Airtightness depends on the membranes, tapes and sealants, and the care taken fitting them.
• Repairs and alterations are usually simple.

SIPs

A SIP is a panel with an insulating core between two boards. Homebuilding & Renovating describes SIPs as structurally strong, excellent for airtightness and quick to build. The insulation is part of the panel, so there is less framing to bridge it.

• Walls arrive as panels. Fewer pieces to fit on site.
• Good airtightness is built in, if the joints are sealed well.
• The core material and thickness set the thermal performance. Ask which core is used.
• Openings are planned at the design stage. Changing them later may be harder than in a frame.
• Moisture detailing still matters. Ask how the joints and the base are protected.

Our source for SIPs is a trade magazine guide, and only part of it is public. We have not found independent test data to compare them with timber frame, so we do not rank them. Ask each builder for the wall specification and the expected thermal performance, in writing.

Modular

A modular garden room is made in a factory, often in one or more sections, then moved to site. The walls inside can be timber frame, SIPs or something else.

• Less work on site. Less time on your lawn.
• Needs a route for delivery, and often a crane or lifting kit.
• Size is limited by what can be moved by road and into your garden.
• You still need a prepared base.

Access is the main question. Measure side passages, gates and overhead cables. A timeline should include delivery day. If the garden has no access, ask for a panelised build, with parts carried in.

The rules do not change with the build type

Planning

Permitted development depends on size, height, position and use. The Planning Portal says outbuildings must be single storey, with eaves no higher than 2.5 m, and an overall height of up to 4 m with a dual-pitched roof or 3 m with any other roof. Within 2 m of a boundary the limit is 2.5 m. Wall type does not enter into it. See planning permission.

Building regulations

The Planning Portal says a building under 15 m² with no sleeping accommodation is normally exempt. Between 15 m² and 30 m² it is normally exempt if it is at least 1 m from any boundary, or built substantially of non-combustible materials. That second route is about materials. Ask the builder and building control how your wall build-up is treated. See building regulations for garden rooms.

Questions to ask any builder

• What is the wall made of, layer by layer?
• What is the roof and floor build-up?
• How is the room kept airtight, and how is it ventilated?
• Where is it built, and how does it reach my garden?
• What warranty covers the structure?
• Can I see a finished room?

Compare these on the quotes, not on the sales language. See bespoke vs flat-pack for another split.

Does it matter for the use

For an office or studio, comfort, quiet and warmth matter most. For a gym, floor strength and ventilation do. Think about those more than the label. For a sleeping annexe, the regulations apply whatever the build. See garden annexe rules.

Cost

We do not give prices here. Methods differ in materials, labour and delivery. Use the garden room cost calculator and read garden room costs.

Local builders

Builders in Berkhamsted, Harpenden, Chesham and Beaconsfield use different methods. Our free matching gives you up to 3 to compare.