Flat, pitched or roof lantern?
A flat roof with rooflights is the usual choice for a kitchen extension: it costs least, fits most easily under the windows above, and simple to keep watertight. A pitched roof matches the house but needs more height at the back wall. A roof lantern on a flat roof lets in the most light and costs the most.
What each roof costs relative to the others, how much light it lets in, and the height rules.
Free, no obligation. One local builder prices it, not five.
- 0.20W/m²K: flat roof U-value for an extension
- 0.16W/m²K: pitched roof
- 1.4W/m²K: rooflights and lanterns
- Eavesthe highest a flat roof may go under the exemption
Flat roof with rooflights
A flat roof isn't flat: it falls gently to a gutter or outlet. Under the exemption it can be no higher than the eaves or parapet of the house, which on most two-storey houses leaves room for a good ceiling height. It must reach a U-value of 0.20 W/m²K. A single-ply membrane or a fibreglass (GRP) finish are the usual coverings. Rooflights go in the flat roof on upstands.
Pitched roof
A lean-to pitched roof with slates or tiles to match the house. It looks traditional and sheds rain easily, but it needs height where it meets the house, so it can clash with the bedroom windows above. Under the exemption a pitched roof can go no higher than the top of the house roof, and the walls no higher than the back wall. It needs a U-value of 0.16 W/m²K. A vaulted ceiling inside, following the slope, gives a lot of height.
Roof lantern
A lantern is a glazed, pitched structure on a flat roof. It brings light into the middle of a deep room and a sense of height. It sits on a raised kerb, so the flat roof is built around it, and it costs more than rooflights. Like any rooflight it must meet 1.4 W/m²K, and solar control glass helps stop the room overheating in summer.
The three side by side
| Flat with rooflights | Pitched | Flat with a lantern | |
|---|---|---|---|
| Cost | Lowest | Higher | Highest |
| Light into the middle | Good, from 2 to 3 rooflights | Good with rooflights in the slope | Most |
| Height needed at the house | Least | Most | Least, plus the lantern above |
| Insulation (U-value) | 0.20 W/m²K | 0.16 W/m²K | 0.20 W/m²K, glass 1.4 |
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A 25 m² single-storey rear extension in Ireland, mid-range, with wide glass doors and a new kitchen: €62,000 to €90,000 all in, including VAT and fees.
How this is worked out
We start from what single-storey extensions cost to build per square metre in each part of Ireland, including VAT at 13.5%, and the rates for side, wraparound, two-storey and heavily glazed extensions. Standard finishes take the lower half of the range, mid-range the upper half, and high spec or a wall of glass the glazed rate. Then we add professional fees (design, engineer, BER) and, if you want one, a new kitchen at your level of finish. Totals are rounded to the nearest €1,000. It's an estimate to plan with, never a quote: the builder prices your house after seeing it.
Questions people ask
Which roof costs least for a kitchen extension?
A flat roof with a couple of rooflights.
Is a flat roof a problem in Ireland's weather?
Not when it's built with a proper fall, a good membrane or GRP finish and well-detailed upstands. Most leaks come from the edges and rooflight upstands, not the middle.
Do I need planning permission for a roof lantern?
Not if the extension is exempt. A lantern on a flat roof counts as part of the roof; check it stays within the height limits.
How high can my extension roof be without planning?
A flat roof no higher than the eaves or parapet of the house; any other roof no higher than the highest part of the house roof. Walls no higher than the back wall.
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