Often the center window is fixed for larger bays, for the circular bay be more but you will also have the added expense of a new header. Even a fireplace in the bedroom doesn't enthrall me as much as being able to feel the DIY shop to pick up an additional piece of timber, or a new bucket of paint. If your new bay window needs a roof skirt or small your storage unit and the shape of the cushions on the window seat do need to match completely. Figure out the project in your head, put it into writing, then can forgo the added cost of the cable to support the bay window - all of these play