We're having patio doors put in where there are currently two small windows at the back of the house (Edwardian era). The builder isn't FENSA registered, so I think we'd need a separate building regs certificate if we wanted to sell, which he has advised to get. He is a well-respected builder in the local area. We have no plans to sell, is it a huge mistake to not pay the extra to get the certificate? We're on a really tight budget. TIA.