Belfast·SPG004 Residential Extensions and Alterations·Page 19
Permitted development - side extensions
Side extensions allowed without planning permission must be less than half the width of the original house, with restrictions on dimensions and proximity to plot boundaries.
What is allowed without planning permission applies largely to the dimensions of the proposed extension and its proximity to plot boundaries. It should also be less than half the width of the original house.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/belfast/SPG004 Residential Extensions and Alterations.pdf