for buffalo ny:
under my porch is concrete. for a porch or deck think about rainwater,
snow, and a permit. those piers holding up my second floor porch of
your size 10 x 25' are 8"x8" pressure treated sunk 4 feet into the
ground. go look at the neighbors incorrect deck made of 2x4" and make
all yours 2x8" at 16" on center with 2 x 6" deck. make it twice as
sturdy as any carpenter says, and it will be just as sturdy at 20 years
of age as the day it was built.
locally you would in buffalo ny 2 years ago see your local city hall
permit office, phone ahead to see what paperwork you need to take there
such as 5 copies of your survey and permit fee money as required. some
city websites have online permits.

wrote:
Is there any rule of thumb on size of crawl spaces? I am building a
three season 10'x24' porch on piers off the family room. The
family room has a cement foundation with a concrete floor. How much
room do I need between the floor joists on the porch and the dirt below
them? Do I need a vapor barrier? Does the crawl space need and what
size vents?