If the attic space is described in sales information and used for storage then you have no problem.
The engineer is possibly not entirely familiar with the regulations, or has for some reason thought the attic to be a habitable space, or you have a purchaser who is trying to hammer down the price.
A rear facing Velux (or similar manufacturer's) roof light serving an attic store does not require planning permission. This issue has been surrounded by quite a lot of confusion in the past. While this is not stated specifically in the planning regulation’s list of exemptions, an Bord Pleanala has ruled on this issue:
"NOW THEREFORE An Bord Pleanála, in exercise of the powers conferred on it by
section 5 (3) (a) of the 2000 Act, hereby decides that -
(a) the said installation of six number roof lights in the eastern roof plane of the
dwelling house is development and is exempted development, ..."
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You can refer the purchaser, their solicitor etc. to the above case.
You have created a storage space not a habitable space, therefore:
Fire regulations (Part B) do not apply,
Ventilation regulations (Part F) do not apply - (Where heights are regulated),
Staircase Requirements do not apply (Part K), as you have created a store, and therefore could have installed only a ladder.
Structural requirements (Part A) are not applicable as you are merely storing stuff in an attic, which is something that is routinely done. (A proper attic conversion designed as habitable space would normally require ceiling joists to be strengthened to carry additional loading).
Insulation & Ventilation requirements (to prevent condensation within the roof structure) remain unchanged, -there is no requirement to upgrade roof insulation when flooring out an attic for storage (although it would be beneficial to do so).
In short, and from the information you have provided, your attic work to provide storage space does not contravene any Building Regulations or Planning Regulations (unless the house is a Protected Structure).
DBK100 - MESH Architects
http://www.mesh.ie/