Hi kaur8888
Yes you can convert the window to a bay, before any changes a survey would need to be carried out.
Remember structural safety if the replacement windows are wider than those they replace or support the upper floor.Bay windows now have reinforcement supports built in when required for support.
Use an installer registered under the FENSA scheme run by the Glass and Glazing federation, in which case you do not need to make a Building
Regulations application to the Council. Your FENSA installer should then ensure your windows comply fully with the regulations and will then register the installation with FENSA who will then supply you with a certificate confirming this when the installation is complete. If you wish to use this option please ensure that your installer is properly registered under the scheme before placing an order. You can check this and find more details of the scheme by visiting the FENSA website at www.fensa.co.uk
As for costs this depends on your requirements.
Hope this helps
Trevor & Mike