International Building Code Section 1209

International Building Code 1209.1

Crawl spaces. Crawl spaces shall be provided with a
minimum of one access opening not less than 18 inches by 24
inches (457 mm by 610 mm).

International Building Code 1209.2

Attic spaces. An opening not less than 20 inches by 30
inches (559 mmby 762 mm) shall be provided to any attic area
having a clear height of over 30 inches (762 mm). A 30-inch
(762 mm) minimum clear headroom in the attic space shall be
provided at or above the access opening.

International Building Code 1209.3

Mechanical appliances. Access to mechanical appliances
installed in under-floor areas, in attic spaces and on roofs
or elevated structures shall be in accordance with the International
Mechanical Code.