No; 4m is the maximum single-storey rear extension allowed for a detached property under the permitted development rules.
Google 'Harrow Residential Design Guide SDP' for their planning policy. Page 62 suggests it can be more flexible depending on site circumstances/proximity to neighbouring properties etc.
Google 'Harrow Residential Design Guide SDP' for their planning policy. Page 62 suggests it can be more flexible depending on site circumstances/proximity to neighbouring properties etc.