Revises the definition of "substantial development," for purposes of the shoreline management act, to exclude the construction or placement of an additional dwelling on a lot if the lot is already occupied by a principal residence.

Requires an additional dwelling to be used by a person: (1) Who is to receive from or administer to a resident of the principal residence, continuous care and assistance necessitated by advanced age or infirmity; or

(2) Over sixty-two years old who is related by blood or marriage to a resident of the principal residence.