Will I need building regulations approval for my bay or bow window?

Building a new bay window is classed as an extension, so you will need building regulations approval.

A bow window is simply a curved window and doesn't increase the floor area and is not classed as an extension.

If you are simply replacing a bay window the same rules apply as for any replacement window, and you will need building regulations approval unless you are using a competent person (use the search above to find one).

Further information

Building regulations applications FAQs

(Image above courtesy of https://shutters.co.uk/)