Installing a Velux window into an existing roof is regarded as a material alteration because it is technically a structural and or thermal alteration. An external window, however, is classified as a “controlled fitting” under the Building Regulations, and the standards outlined with regards to it must be met. To avoid confusion, use a registered installer and a professional loft conversion specialist like Clifton Hill & Montpelier Loft Conversions. We are approved to carry out the work in such a way that it complies with building regulations without having to involve any local authority building control. You will get a certificate after the work is completed stating that the work was done by a registered installer and also, your fees will be cheaper as you are removing building control fees.