correct 3 bay victorian conservatories are at 45 degrees - or more accurately at 135 degrees

there are lots of trade software around - such as
http://www.roofwright.co.uk

however I imagine lots of the relatively low tech home design packages available on the web will be sufficient for mosy laypersons requirements.

have not tried this - but it looks promising: http://www.artifice.com/dw_classic.html

Do allow however for "thickness" of wall when calculating internal floor area

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Twenty years from now you will be more disappointed by the things you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover. - Mark Twain