As long as House B is a separate entity in the title it can avail of the exemption and as long as you can fit the criteria in the below. Here is the Irish Statue regulations.
Part 1
Exempted Development — General
Column 1
Description of Development
Column 2
Conditions and Limitations
Development within the curtilage of a house
CLASS 1
The extension of a house, by the construction or erection of an extension (including a conservatory) to the rear of the house or by the conversion for use as part of the house of any garage, store, shed or other similar structure attached to the rear or to the side of the house.
1. (a) Where the house has not been extended previously, the floor area of any such extension shall not exceed 40 square metres.
(b) Subject to paragraph (a), where the house is terraced or semi-detached, the floor area of any extension above ground level shall not exceed 12 square metres.
(c) Subject to paragraph (a), where the house is detached, the floor area of any extension above ground level shall not exceed 20 square metres.
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.
(b) Subject to paragraph (a), where the house is terraced or semi-detached and has been extended previously, the floor area of any extension above ground level taken together with the floor area of any previous extension or extensions above ground level constructed or erected after 1 October 1964, including those for which planning permission has been obtained, shall not exceed 12 square metres.
(c) Subject to paragraph (a), where the house is detached and has been extended previously, the floor area of any extension above ground level, taken together with the floor area of any previous extension or extensions above ground level constructed or erected after 1 October 1964, including those for which planning permission has been obtained, shall not exceed 20 square metres.
3. Any above ground floor extension shall be a distance of not less than 2 metres from any party boundary.
4. (a) Where the rear wall of the house does not include a gable, the height of the walls of any such extension shall not exceed the height of the rear wall of the house.
(b) Where the rear wall of the house includes a gable, the height of the walls of any such extension shall not exceed the height of the side walls of the house.
(c) The height of the highest part of the roof of any such extension shall not exceed, in the case of a flat roofed extension, the height of the eaves or parapet, as may be appropriate, or, in any other case, shall not exceed the height of the highest part of the roof of the dwelling.
5. The construction or erection of any such extension to the rear of the house shall not reduce the area of private open space, reserved exclusively for the use of the occupants of the house, to the rear of the house to less than 25 square metres.
6. (a) Any window proposed at ground level in any such extension shall not be less than 1 metre from the boundary it faces.
(b) Any window proposed above ground level in any such extension shall not be less than 11 metres from the boundary it faces.
(c) Where the house is detached and the floor area of the extension above ground level exceeds 12 square metres, any window proposed at above ground level shall not be less than 11 metres from the boundary it faces.
7. The roof of any extension shall not be used as a balcony or roof garden.