Hi Lady Louisa,
You are best advised to advise your client full.
Sad to see an engineer's apparent incompetence to certify planning matters.
Before lodging any planning applciation, I would become thoroughly familiar with the previous refused application.
25 sqm is the current, not historical exempted development for a garage - please see the relevant quotation below.
I would be wary about seeking a section 5 declaration for the house extension if it is in fact exempted development.
I would neither seek not issue a cert on it at all yet.
I would seek retention for the garage it is only 10 sqm oversized,
In fact if you wanted to box clever you could apply for the additional square meterage
If you get permission for the garage - and this may require a plnning appeal - I would go back and address the extension.
There are other factors affecting whether or not something may be exempted, but the planning application process should shake those apples out of the tree.
However - and this is a big however - you must absolutely check the exempted status of the existing house extension.
That comes under CLass 1 of the below regulations, but these have been amended in recent time, particularly Class 50.
Please see the Planning and Development Regulations 2008 - S.I. 235 of 2008 http://www.irishstatutebook.ie/2008/en/si/0235.html
There are a lot of other factors to check, including the development plan zoning, special objectives, listed views, protected structure status.
One possible issue arises if the house is semi-detached and the outer wall of the extension is built on top of the party boundary.
While development within the curtiledge of a house is exempted, once it goes over the boundayr in theory it may no longer be exempted.
There is whole host of other requirements to do with eaves heights and roofs, for example if the development is flat-roofed to the rear of a bungalow.
This quite detailed so I'm going to cover myself here and say;
All advice on Archiseek is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.
The construction, erection or placing within the curtilage of a house of any tent, awning, shade or other object, greenhouse, garage, store, shed or other similar structure.
1. No such structure shall be constructed, erected or placed forward of the front wall of a house.
2. The total area of such structures constructed, erected or placed within the curtilage of a house shall not, taken together with any other such structures previously constructed, erected or placed within the said curtilage, exceed 25 square metres.
3. The construction, erection or placing within the curtilage of a house of any such structure shall not reduce the amount of private open space reserved exclusively for the use of the occupants of the house to the rear or to the side of the house to less than 25 square metres.
4. The external finishes of any garage or other structure constructed, erected or placed to the side of a house, and the roof covering where any such structure has a tiled or slated roof, shall conform with those of the house.
5. The height of any such structure shall not exceed, in the case of a building with a tiled or slated pitched roof, 4 metres or, in any other case, 3 metres.
6. The structure shall not be used for human habitation or for the keeping of pigs, poultry, pigeons, ponies or horses, or for any other purpose other than a purpose incidental to the enjoyment of the house as such.