De minimis levels are the amounts of painted surfaces to be disturbed during rehabilitation, maintenance, paint stabilization or hazard reduction activity, below which safe work practices and clearance are not required.

24 CFR 35.1350
  (d) De minimis levels. Safe work practices are not required when maintenance or hazard reduction activities do not disturb painted surfaces that total more than:
  (1) 20 square feet (2 square meters) on exterior surfaces;
  (2) 2 square feet (0.2 square meters) in any one interior room or space; or
  (3) 10 percent of the total surface area on an interior or exterior type of component with a small surface area. Examples include window sills, baseboards, and trim.

24 CFR 35.1340
  (g) Worksite clearance. Clearance is not required if maintenance or hazard reduction activities in the worksite do not disturb painted surfaces of a total area more than that set forth in Sec. 35.1350(d).

Interpretive Guidance
Notice of Visual Assessment and Hazard Control Below the De Minimis
Clearance and De Minimis
De Minimis Levels