As woody suggests - do you know what you want to build but just need a nice way of drawing it up, or are you trying to convert your requirement for a '4m x 3m single storey pitched roof rear extension' into a generic extension design with detail like "100mm cavity, 600mm wide strip foundation, C16 joists blah blah"?
The latter could be tricky but there's enough knowledge on this forum to get you there.
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