International Residential Code - 2006
Access shall be provided to all under-floor spaces. Access openings through the floor shall be a minimum of 18 inches by 24 inches (457 mm by 610 mm).
Openings through a perimeter wall shall be not less than 16 inches by 24 inches
(407 mm x 610 mm).
When any portion of the through-wall access is below grade, an areaway not less than 16 inches by 24 inches (407 mm x 610 mm) shall be provided. The bottom of the areaway shall be below the threshold of the access opening.
Through wall access openings shall not be located under a door to the residence.
See Section M1305.1.4 for access requirements where mechanical equipment is located under floors.
M1305.1.4 Appliances under floor
Underfloor spaces containing appliances requiring access shall have an unobstructed passageway large enough to remove the largest appliance, but not less than 30 inches (762 mm) high and 22 inches (559 mm) wide, nor more than 20 feet (6096 mm) long when measured along the centerline of the passageway from the opening to the appliance. A level service space at least 30 inches (762 mm) deep and 30 inches (762 mm) wide shall be present at the front or service side of the appliance. If the depth of the passageway or the service space exceeds 12 inches (305 mm) below the adjoining grade, the walls of the passageway shall be lined with concrete or masonry extending 4 inches (102 mm) above the adjoining grade in accordance with Chapter 4. The rough-framed access opening dimensions shall be a minimum of 22 inches by 30 inches (559 mm by 762 mm),where the dimensions are large enough to remove the largest appliance.
1. The passageway is not required where the level service space is present
when the access is open, and the appliance can be serviced and removed
through the required opening.
2. Where the passageway is unobstructed and not less than 6 feet high
(1929 mm) and 22 inches wide for its entire length,the passageway
shall not be limited in length.