NFPA 58 Liquefied Petroleum Gas Code written by the National Fire Protection Association which includes Propane, states:
Chapter 5: Storage of Cylinders Awaiting Use,
Resale, or Exchange
5.1.1.1 The provisions of this chapter shall apply to the storage
of cylinders of 1000-lb (454-kg) water capacity or less,
whether filled, partially filled, or empty as follows:
5.3.4 Storage Within Residential Buildings. Storage of cylinders
within a residential building, including the basement or
any storage area in a common basement storage area in multiple-
family buildings and attached garages, shall be limited to
cylinders each with a maximum water capacity of
2.7 lb
(1.2 kg) and shall not exceed 5.4-lb (2.4-kg) aggregate water
capacity for smaller cylinders per each living space unit. Each
cylinder shall meet DOT specifications.
5.4 Storage Outside of Buildings.
5.4.1 * Location of Storage Outside of Buildings. Storage outside
of buildings for cylinders awaiting use, resale, or part of a
cylinder exchange point shall be located as follows.
5.4.1.1 At least 5 ft (1.5 m) from any doorway or opening in a
building frequented by the public where occupants have at least
two means of egress as defined by NFPA 101, Life Safety Code. For
buildings, or sections of buildings, having only one means of
egress, at least 10 ft (3m) from the doorway or opening.
Since tanks used for Grills etc. are at least 20 lb aggregate water capacity they should NEVER under any circumstances be stored inside!
THIS IS THE NATIONAL CODE! Some states have developed revisions to the National Code which are even stricter.