Answer in brief
A lean-to kit suits a house with enough wall height between the ground-floor ceiling and the sills of the upstairs windows. The roof needs a slope steep enough for the covering you choose, and the low edge has to clear the 3 m eaves limit near a boundary. Where the wall is too short, a flat roof kit is the better fit.
A lean-to is the simplest pitched roof there is: one slope, high against the house and low at the garden end. Kit makers like it because the rafters are all the same, and homeowners like it because it looks at home on a brick house in a way a flat roof sometimes does not. It does not suit every house, though, and the reason is nearly always height.
The height problem
The top of a lean-to roof meets the back wall of the house. Above that junction you need room for flashing and, on most houses, the upstairs window sills sit not far above. Below it, inside, you need a decent ceiling height at the low end of the room.
Work it out with a tape measure before you look at kits:
- Measure from the ground to the bottom of the lowest upstairs window sill on the back wall.
- Take off a margin for the flashing and a clear gap under the sill.
- That is the highest the roof can meet the wall.
- From that point, run the slope out to the depth you want and see where the low edge lands.
On a 3 m deep room, a few degrees of pitch either way moves the low edge by a surprising amount. If the low edge ends up below about head height inside, the room will feel cramped at the garden end and the doors will be squat.
Roof pitch and covering
Each roof covering has a minimum pitch set by its maker. Interlocking concrete or clay tiles generally need more slope than natural slate, and metal sheet or lightweight tile systems can go flatter still. Choose the covering first, then check the pitch it needs against the height you measured. A kit supplier will ask for both.
Glazed lean-to roofs, the conservatory kind, can be very shallow, but a fully glazed roof struggles to meet the insulation standards a kit room has to meet. Most kit lean-tos are solid roofs with one or two rooflights.
The planning limits
England’s permitted development height cap for a one-storey back extension is 4 m at the top, which for a lean-to is the line where it meets the house. Closer than 2 m to a boundary, the eaves on that side are capped at 3 m. The eaves of a lean-to are its low edge, so this is rarely a problem at the garden end, but check the side walls, which slope.
Projection limits do not change for a lean-to: 3 m from an attached house, 4 m from a detached one, further with prior approval.
Lean-to or flat roof?
| Lean-to kit | Flat roof kit | |
|---|---|---|
| Look against a brick house | Traditional, matches tiles | Modern, crisp lines |
| Height needed on the house wall | More | Less |
| Ceiling inside | Slopes, higher at the house | Level throughout |
| Roof covering | Tiles, slate or metal | Membrane such as EPDM or GRP |
| Roof lanterns | Rooflights in the slope | Lantern or flat rooflights |
A lean-to costs a little more than a flat roof of the same footprint, because of the rafters and the tiles. Where the wall height allows it, many people find it worth the difference.
Before you order
Send the supplier the sill height, the depth you want and the roof covering you prefer. A good one will come back with a section drawing showing the inside height at the low end. If you are still choosing a footprint, our guide to prefab extension kits shows the common sizes and what each costs.