Planning exemption

The obvious question is why PP was needed for the 16m2 in the first place. Im assuming its because there was some condition of the exemptions that couldnt be complied with and hence the need to seek permission. That being the case then you would need to build in accordance with the PP that was granted.

On the other hand and to be more precise, yes the 16m2 will eat into the exempted allowance. The following paragraph is taken from Schedule 2 here.

2. (a) Where the house has been extended previously, the floor area of any such extension, taken together with the floor area of any previous extension or extensions constructed or erected after 1 October 1964, including those for which planning permission has been obtained, shall not exceed 40 square metres.

In that situation, yes, but that isn't the case here. It's a rear extension to a detached house.

It could also be the case that they weren't aware of the exemption and/or received very poor advice.

also needs to be 2m clear.

• Detached Houses: As with terraced and Semi-detached above, the overall area of extension must be less than 40 sq. metres. This exemption also allows for extensions above ground floor level, the are above ground floor are cannot exceed 20 sq. m. One could therefore have a ground floor extension of 20 sq. m., and a 2nd floor of 20 sq. m.  Any above ground floor extension shall be a distance of not less than 2 metres from any party boundary.

also requires there to be 25 sq m of open space left.

@Ferney is it 2m away from the boundary? will you have 25 m sq open house and what type of roof will it have? is there windows overlooking neighbours and if so what distance are they

If details are as OP says it’s straight forward and there’s no need for a S.5

it’s only €80 helps if he ever sales and provides clarity