Belfast·SPG004 Residential Extensions and Alterations·Page 24·4.1.18
Single storey versus two storey rear extensions
Single storey rear extensions have less impact on neighbours than two storey extensions. Two storey extensions are more likely acceptable on detached houses and may be possible on larger terraces or semi-detached houses with stepped-back upper storeys.
Single storey rear extensions and those which are shorter in length will generally have less of an impact on neighbouring amenity than extensions which are longer or which comprise two storeys. Two storey rear extensions are more likely to be possible on detached houses, but may be possible for larger terraces and semi-detached houses, particularly where the second storey extension is stepped back from the boundary.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/belfast/SPG004 Residential Extensions and Alterations.pdf