Construction approved prior to this chapter: Nothing in this chapter shall be deemed to require a change in the plans, construction or designated use of any building on which actual construction was lawfully begun prior to the effective date of this chapter and upon which actual building construction has been carried on without interruption, except to causes beyond the control of the developer. Actual construction is hereby defined to include the placing of construction materials in permanent position and fastened in a permanent manner. Where excavation or demolition or removal of an existing building has been substantially begun preparatory to rebuilding pursuant to a validly issued building permit, such excavation or demolition or removal shall be deemed to be actual construction, provided that work shall be carried on without interruption except due to causes beyond the control of the developer.