Section 81R. A planning board may in any particular case, where such action is in the public interest and not inconsistent with the intent and purpose of the subdivision control law, waive strict compliance with its rules and regulations, and with the frontage or access requirements specified in said law, and may, where the ways are not otherwise deemed adequate, approve a plan on conditions limiting the lots upon which buildings may be erected and the number of buildings that may be erected on particular lots and the length of time for which particular buildings may be maintained without further consent by the planning board to the access provided. The planning board shall endorse such conditions on the plan to which they relate, or set them forth in a separate instrument attached thereto to which reference is made on such plan and which shall for the purpose of the subdivision control law be deemed to be a part of the plan.