BUTTERFLY ROOFS In some terraces the historic butterfly roof pattern survives unaltered. In such cases, roof extensions will normally be unacceptable. However, if the terrace has been altered and includes other roof extensions, a mansard extension may be acceptable but the "V" shaped parapet wall should be retained. Gap between two terraces. Mansard with butterfly parapet retained at rear. http://www3.westminster.gov.uk/spgs/publications/Roof%20guide.pdf p21