The roof is the part of an extension that decides how the room feels and how much of the planning envelope it uses, and it is usually chosen last when it should be chosen early. Three families cover almost every domestic extension: a flat roof, a pitched roof, and a roof with substantial glazing in it.
A flat roof single storey extension, and why most have one
A flat roof is the cheapest, takes the least height, and leaves the most room under the permitted development limits, which matters because a rear extension has a maximum eaves and overall height and a pitched roof eats both. It also lets the ceiling inside be flat and high right to the back wall. Modern flat roofs on warm deck construction with a single ply or bitumen membrane are long life coverings rather than the failure prone things people remember, and they take rooflights easily, which is how light reaches the middle of a deep plan.
Pitched roofs, and what they buy
A pitched roof reads as part of the house, sheds water without relying on a designed fall, and allows a vaulted ceiling inside if the structure is designed for it, which transforms a small room. It costs more, takes more height, and on a rear extension the height is frequently what pushes the design into a planning application. A lean to or mono pitch falls one way against the house and is the compromise: some of the character, much less height than a gable or a hipped roof.
An extension with glass roof, or rooflights
A fully glazed extension roof gives the most light and the most heat, both in and out. Modern units with solar control and low emissivity coatings have made this far more viable than it was, and a large south or west facing glass roof still needs shading and ventilation designed in rather than discovered. For most extensions the better value answer is an insulated roof with generous rooflights or a lantern: close to the same daylight where it is wanted, far better thermal performance, and far less glass to clean on a surface nobody can reach.
Glass roof extension cost, pitched or flat: what the choice does to the price
A flat roof is the cheapest per square metre and the pitched roof adds structure, covering and often a ceiling to build. Glazing is the most expensive per square metre by a distance. But the roof choice also changes the structure below it, because a pitched roof loads the walls differently, and it changes the planning route. So the honest comparison is not the roof alone: it is the roof plus its effect on the structure, the approvals and the finished ceiling height.
Building over an existing flat roof
Adding a first floor over an existing single storey flat roofed extension is common and it is not an extension of that roof: it is a new storey, and the question is whether the existing foundations and walls were built for it. Frequently they were not, and the cost is in the underpinning or rebuilding rather than in the new floor. Get that assessed before pricing anything, because it is the difference between a straightforward job and a rebuild.
Questions people ask about extension roof types
Is a flat roof or a pitched roof better for an extension?
A flat roof is cheaper, uses less of the height allowance and takes rooflights easily. A pitched roof looks more like part of the house and allows a vaulted ceiling. Height against the permitted development limits is usually what decides it.
Do flat roof extensions still leak?
A modern warm deck roof with a proper fall and a good membrane is a long life covering. The failures come from falls that pond and from details at upstands and outlets, not from flatness itself.
Is a glass roof worth it on an extension?
For maximum light, yes, and it costs the most and needs shading and ventilation planned in. An insulated roof with generous rooflights usually gives most of the daylight for much better thermal performance.
Can I build a first floor over my flat roof extension?
Only if the existing foundations and walls were built to carry it, which frequently they were not. That assessment decides the cost, so get it done before anything else is priced.