Officials in San Francisco plan to look into the city’s growing vacation rental business.

Some landlords are signing short-term leases with visiting tourists who are just passing through the city, reported the Bay Citizen. There are hundreds of these rentals in San Francisco, and nearly all of them could be considered illegal because their landlords did not seek prior approval from the city, according to the report.

A seldom-used 30-year-old city ordinance requires that property managers seek city approval before offering short-term leases. But since 2004, only one unit in the entire city has actually received that permission, officials at the Department of Building Inspection told the paper.

There is concern among some city officials that if many of the city’s residential units are reserved for tourists, fewer will be available for San Francisco residents that need housing. One expert told the paper that as many as 1,000 units had been illegally converted into corporate housing or vacation rentals.

Property managers should be sure to make themselves aware of all relevant city ordinances before making any alterations to their apartment units or traditional leasing arrangements.