24 CFR Part 100—Fair Housing Accessibility Guidelines, Preamble

(c) Thresholds at patio, deck or balcony doors.

The final Guidelines provide that exterior deck, patio, or balcony surfaces should be no more than 1/2 inch below the floor level of the interior of the dwelling unit, unless they are constructed of impervious materials such as concrete, brick or flagstone, in which case the surface should be no more than 4 inches below the floor level of the interior dwelling unit, unless the local building code requires a lower drop. This provision and the following provision were included in order to minimize the possibility of interior water damage when exterior surfaces are constructed of impervious materials.