In normal circumstances, you can replace or add new UPVC windows to your home without needing planning permission. However, if your building is listed or in a conservation area you are likely to require planning permission.
Typically, if you are replacing windows with a similar design, then planning permission is not required in areas where the council has not put any restrictions in place. However, there may be some conditions that you will need to fulfil, and your local installer will be able to provide advice about whether any aspects of your chosen design require approval.
In some areas, adding a bay window to a home would count as an extension, in which case, you would need to check whether permission is needed for the change. Your installer will be familiar with local guidelines and can help you through the process of choosing windows and doors that will improve your home.