Legal Question in Real Estate Law in California

Without knowing more details of the city's grounds for the requirement, my guess is that you have a zoning, building code and land use issue coupled with a possible misunderstanding. Is your dad using the portables, or just storing them there for use on job sites? If he is using them, the city may be right. Many cities require portables that are in-use/occupied, to be set on foundations just like mobile homes. This is considered a safety issue. You can argue whether that makes any sense, but if it is the law, it is enforceable. The fencing is probably a similar issue, though perhaps not related to whether they are in use or not. Portables on commercially zoned properties are magnets for vandals, druggies, squatters, you name it. That is a burden on the local law enforcement. So they probably have a requirement that portables be in a fenced space if on commercial land, to reduce the ease with which unsavory types could make them a problem.

Cities generally cannot make demands based upon esthetic judgments. They can enforce zoning and building codes and other ordinances with a rational and objective basis. You or your father should get a copy of the zoning ordinance governing this particular parcel and see if you are in compliance; also, try asking the city to cite chapter and verse of the law or regulation with which you aren't in compliance. At some point, however, you may have to face the possibility of spending the money to comply. A local attorney with recent experience assisting developers, builders and others with "fighting city hall" in your town may be able to give you some insights based on personal experience and local connections.