Is the 40 square metre planning exemption cumulative in Ireland?

The 40 m² exemption for house extensions under Class 1 of Schedule 2 to the Planning and Development Regulations 2001 is measured cumulatively. This means the limit applies to the combined floor area of every extension added to the original house since it was first built, not to each new extension considered in isolation. If a previous owner already added a 20 m² extension under the exemption, only 20 m² of exempted floor area remains available for any future extension.

This cumulative approach applies whether the earlier extension was built under the exemption, under a specific grant of planning permission, or even without any permission at all — the regulations look at the total area added to the 'original house' as it stood on the relevant appointed date, generally 1 October 1964 or the date the house was built if later. Extensions that were the subject of their own planning permission are usually excluded from the cumulative calculation, but this needs to be checked against the specific permission granted.

In practice this means a homeowner planning a new extension must establish, often through old planning files, historic Ordnance Survey mapping, or the property's own planning history with the local authority, exactly how much floor area has already been added under the exemption. Getting this wrong and exceeding the true cumulative limit means the new extension is not exempted development, even if it looks identical to previous exempt work.