I do not know if there is a rule of thumb, we are adding about 700 sq ft, brushed concrete pad, and aluminum-screened enclosure, along with the rearrangement of the sprinklers, and electrical receptacles. Cost is on the order of $17,000 to $20,000 depending on builder. There will be additional costs after the BC is built, landscaping, painting of concrete, etc.
Be aware that as of June 30th Sumter County updated building codes. In our case it did not make any significant changes, except to delay the start 2-3 weeks.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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