Most people choosing a single-storey extension spend their time on the plan — where the island goes, how big the table is, whether the utility room fits. The plan matters, but the roof is what you will actually feel when you stand in the finished room.
The roof decides the ceiling height, where the daylight comes from, what the room feels like at the far end away from the garden doors, and how the new work reads against the old house from outside. It also has more effect on the planning position than almost any other design decision, because height and eaves are what neighbours and planning officers look at.
Here are the six forms we use most, and what each one does.
1. Flat roof
A flat roof — in practice, a shallow fall concealed behind an upstand or parapet — is the most useful roof available for a rear extension, and it is badly underrated because people remember the failed felt roofs of the 1970s. A modern warm-deck flat roof with a single-ply or liquid membrane is a different product entirely.
What it does to the room: gives you the maximum ceiling height for a given external height. Because the whole roof plane is at the same level, the room feels consistent from front to back rather than pinching down at the edges.
Daylight: rooflights can go anywhere, in any shape, without the geometry problems a pitched roof creates. This is the form that lets you bring light deep into the middle of a plan, which is the real problem in most extended terraced and semi-detached houses.
Where it wins: under a first-floor window, where you need to keep the roof low. On a house where a pitched addition would look bolted on. Where you want a parapet and a crisp horizontal line.
The caution: detailing. Upstands, thresholds, outlets and overflows have to be right. A flat roof that is designed properly lasts; one that is drawn as a line on a plan does not.
2. Mono-pitch, or lean-to
A single slope, high against the house and low at the outer edge. This is the traditional back-addition roof and it is still the most economical pitched form.
What it does to the room: you can leave the ceiling open to the slope, which gives a rising volume with the height where you want it, next to the existing house, and the low edge at the garden wall. It reads as generous without needing much external height.
Daylight: the high wall against the existing house is the opportunity. A run of high-level glazing there, or rooflights set into the upper part of the slope, throws light back into the original rooms rather than only into the extension.
Where it wins: almost anywhere the existing house has room under the first-floor windows for the high end. It is the natural companion to a two-storey house.
The caution: the abutment. Where the slope meets the existing wall, the flashing and the interruption of any existing window need thinking about at design stage, not on site.
3. Gable-ended pitched roof
A conventional double-pitched roof with the gable facing the garden.
What it does to the room: the biggest volume of the six. Opened up to the ridge, it turns a kitchen into a room with genuine vertical presence, and it is the form that suits a long dining table under a ridge line.
Daylight: the glazed gable is the point. A full-height glazed end wall under an exposed ridge gives you the view and the light in one move, and no other form does it as well.
Where it wins: detached and semi-detached houses with garden depth. Rural and village settings, where a pitched gable reads as a familiar farm-building form rather than a modern intrusion.
The caution: it is the tallest option at the outer edge, which is exactly where neighbours notice. On a boundary or close to one, expect the height to be the thing that is argued about. Overheating also needs planning for, because a south-facing glazed gable collects a great deal of solar gain.
4. Hipped pitched roof
The same pitched roof, but with the outer end sloped back rather than finished as a gable.
What it does to the room: less internal volume than a gable, because the ceiling closes in from three sides rather than one. If you plaster the ceiling flat you lose the benefit entirely and end up with a pitched roof that behaves like a flat one.
Daylight: harder. Rooflights work, a glazed gable does not exist, and the hipped end reduces the wall area available for tall glazing.
Where it wins: on a house that is itself hipped, where a gabled addition would look wrong. Hipping the roof also reduces the apparent bulk at the far end, which can be the difference in a tight neighbour relationship or a sensitive setting.
The caution: do not choose it for the room. Choose it for the elevation, and accept the internal compromise knowingly.
5. Crown roof
A pitched roof with the top cut off — slopes on the outside, a small flat deck in the middle. It is a hybrid, and a genuinely useful one.
What it does to the room: you get the sloped ceiling around the perimeter with a flat centre, which reads as a tray or coffered ceiling. The overall external height stays lower than a full pitch.
Daylight: the flat centre takes a lantern or a large rooflight, dropping light into the middle of the plan while the slopes keep the outside looking like a pitched roof.
Where it wins: where the neighbours or the local plan will not tolerate a full ridge height, but a flat roof would look wrong on the house.
The caution: more junctions, therefore more cost and more detailing. It is not the cheap option.
6. Fully glazed roof
A glass roof, a full lantern, or a glazed link connecting old to new.
What it does to the room: transforms it. A glazed link between an existing house and a new wing is one of the most effective moves available, because it separates the two buildings visually while joining them physically — which is also why it works so well on listed and period properties.
Daylight: more than you probably want, and light from directly overhead is a different quality from side light. It flattens shadows and reveals every imperfection in the finishes below.
Where it wins: links and thresholds. Deep plans where nothing else will reach the middle.
The caution: heat. A large area of glazed roof will overheat in summer and lose heat in winter unless the specification, orientation and ventilation are all designed together. Solar control glass, opening vents and some shading are not optional extras on a glazed roof, they are part of making the room usable in July. Building regulations now take overheating seriously, and the requirements should be checked for your specific design.
Comparing them
| Roof form | Internal volume | Best light source | External height at the far end | Watch for |
|---|---|---|---|---|
| Flat | Consistent, moderate | Rooflights anywhere | Lowest | Waterproofing detail |
| Mono-pitch | Rising towards the house | High-level glazing at the abutment | Low | Junction with existing wall |
| Gable | Greatest | Glazed gable | Highest | Neighbour impact, solar gain |
| Hipped | Reduced at the edges | Rooflights only | Medium | Ceiling closes in |
| Crown | Perimeter slope, flat centre | Central lantern | Medium | Cost and junctions |
| Glazed | Varies | Overhead, abundant | Varies | Overheating and glare |
How the roof affects the planning position
Height, eaves height and proximity to the boundary are the dimensions that matter, both to permitted development and to a planning officer assessing neighbour amenity. We do not publish the current permitted development figures, because they change and a wrong number here would be worse than no number. What is stable is the principle: a lower roof close to a boundary and a taller roof set away from it is the arrangement that gets consented, and the roof form is how you achieve it.
It is also worth saying that the roof is where an extension either belongs to the house or does not. Matching the pitch and the eaves line of the original is one legitimate approach. Deliberately contrasting — a low flat-roofed box against a pitched Victorian rear — is another, and on period buildings it is often the stronger argument, because the new work is honest about being new.
Seeing it applied
Our rear extensions at Cringleford and the combined loft conversion and single-storey extension at Strumpshaw show how the roof decision follows from the existing house rather than from a catalogue. More on how we handle extension projects is on our house extensions page, and our general design approach is set out under architectural design.
Decide the roof early
The roof form should be settled at concept design, because it determines the structure, the glazing, the ceiling and half the cost. Our free initial consultation is the right place to start: we look at the house, the orientation, the neighbours and the constraints, and tell you which forms are realistic before you commit to anything.
Call the Norwich studio on +44 (0) 1508 830 310 or email info@archidite.com.