PPS 7 Addendum Residential Extensions and Alterations·Page 14·A14
Roof type and pitch matching
Extensions that copy the roof type and angle of pitch of the original residential property are more successful; roofing materials should match the original, and flat or mansard roofed extensions to traditional buildings are seldom harmonious unless not open to public views.
An extension or alteration which copies the roof type and angle of pitch of the original residential property will be more successful than those proposals that introduce a completely different type of roof. The roofing material of any pitched roof extension should seek to match that of the original. Flat or mansard roofed extensions to traditional buildings are seldom harmonious. However, they may be acceptable where they are not open to public views.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/PPS 7 - Addendum Residential Extensions and Alterations.pdf