Many of these houses are what planners call legal nonconforming, meaning they could not be approved under the current zoning and other regulations. This status means the structures can remain indefinitely in their existing "footprint," but cannot be enlarged and sometimes they cannot be moved. If the house is destroyed (for any reason), it can be rebuilt in that footprint. There is an assumption with such structures and uses that eventually they will cease to exist and the underlying land will then be developed only under the current ordinances and regulations. If the structure or use is abandoned for a continuous period of time (usually six months), then the legal nonconforming status is lost and cannot be re-established.
Legal nonconforming properties should always have an accurate survey so that the footprint and size of the structure is well defined in an official document. Without a survey or an existing structure, it can be difficult to prove the "footprint." Many planners claim they have never seen a request for legal nonconforming status that actually meets the burden of proof, which can be quite cumbersome. A survey would be the solution, not that expensive and could save the owners a lot of delay and cost should they reconstruct.