Extensions

Fibreglass Extension Roofs

A GRP roof on a 30 to 40 m² rear kitchen extension costs €3,800 to €6,500, and a warm GRP roof over a heated extension of 35 to 50 m² €6,000 to €9,500+.

Over a heated room, the roof is also the insulation. That's the decision that moves the price most.

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What an extension roof costs

Job	Typical cost	What moves the price
GRP on a kitchen extension, 30 to 40 m²	€3,800 to €6,500	The upstand at the house wall, rooflights
Warm GRP roof over a heated extension, 35 to 50 m²	€6,000 to €9,500+	Insulation boards and taller trims
A GRP flat roof, garage or extension	€2,800 to €6,500	Size, a new deck, trims on every edge, the outlet
Warm roof system, a m²	€130 to €190	Insulation boards, vapour control

Warm roof or not

In a warm roof the insulation boards go on top of the deck, and the GRP on top of them: a vapour control layer over the deck, rigid PIR boards to 0.16 W/m²K or better, then the laminate. Over a kitchen or living room, it's the time to do it, because it only happens when the roof is off.

Extensions and planning

Under S.I. No. 338 of 2026, a rear extension can be exempt from planning up to 45 m² of floor area, taken together with earlier extensions (it was 40 m²). Re-covering the roof of an existing extension the same way is exempted development under section 4(1)(h).

What to look at on an extension

Where the roof meets the house wall: an upstand and a flashing, the commonest leak point.
Rooflights or a lantern sealed into the GRP.
The fall to the gutter, and the outlet size.
Ceiling height inside, if insulation is added on top.