What your asking for, I think, is called a box bay window. There are many ways to put these in. If there is a overhang in that area it will normally cost less as they can use that for the roof of the window. If not you need to have a small roof added to the window. If you want to use the bottom portion under the seat for storage that will need to be figured in also.

The window price runs anywhere from $350 to $1000 depending on the size, sale, quality and options such as side windows opening - side and front opening. In Toronto you will want triple pane.

Installation - labor of a single window runs between $150.00 to $ 400.00 depending on how difficult the window is to install and the level of contractor you use.

Roofing would be additional materials running an additional 100 to $300 depending on the material you choose for example a copper roof will cost more than shingled.

As far as time a good contractor and helper can do it in a day.

What if you already have the window, just need seat with storage underneath? Probably w a hinge or cubby space to be cheaper?
Thank you!

Do you want it to the floor like a Romeo and Juliette balcony or just like above.We did one where we put in french doors that swung in and just a curved railing on the outside so we could enjoy our coffee and look at the garden with the doors open.

If you already have the window and recess is it the box or the bay window and does it go to the floor? The ones above are box this is a bay pictured below.

If you need the wall pushed out under the window you would have to call a carpenter/framer. If you are looking to add a seat it depends on the style of the window to tell you how you can accomplish this easily.

I can help you with the bench seating. Are you in Toronto?

Window treatment, bench seating and pillows need to all coordinate.