Anyone can install windows or doors but unless you or your builder is a registered installer you will have to apply for approval from your local building control body. You can use a Building Notice application.
Building control will check the replacement windows or doors to make sure they comply and, if satisfied, issue a completion certificate.
Alternatively you can use an installer registered with the relevant Competent Person scheme.
A registered installer is approved to carry out the work to comply with building regulations without involving local authority building control.
When work is complete you will receive a certificate showing the work was done by a registered installer. (Make sure you have received it as you'll need it when you come to sell your home.)
Do you have questions about applying for building regulations approval? Visit the Building regulations applications FAQs.