If you buy a home in San Francisco and find it has unpermitted alterations, do you have to "legalise" it? According to the Department of Building Inspection, the answer is yes.

Taking on a home with unpermitted alterations is a nightmare because urban housing is expensive, building codes are complicated, and few buyers can afford the unanticipated costs at closing. A buyer who discovers that a home has been previously altered with unauthorised construction is responsible for bringing it into compliance with the code, but the cost of repairs can be quite high.

This is why the City of San Francisco has tried to discourage this type of thing since the beginning. Based on the construction value, homeowners who upgrade their property without a legal permit could end up paying the city up to nine times the cost of the unauthorised work.

"According to Patrick Hannan, a spokesman for the Department of Building Inspection, buyers who undertake unauthorised alterations don't have to pay those fees, but they do have to fix any concerns raised by inspectors, the San Francisco Chronicle reported. He pointed out that inspections are done on the basis of complaints, not at the time of sale, so licensing issues may go unnoticed.

For minor issues, the inspector will ask the owner to address them. For larger concerns that pose a safety risk, the inspector will issue a notice of violation stating what the owner must do to bring the property back into compliance, including obtaining the final period of time needed to proceed with the construction permit.

The goal of the inspections is to make the property legal, not to penalise unintentional new homeowners, and no fines will be levied as long as the problem is resolved, Hannan said. Homeowners who "voluntarily" seek to remedy unauthorised work, rather than react to a breach, are not penalised. He encourages buyers to use the inspections department's Permit and Complaint Tracking System, which lists complaints and building code violations going back decades, when researching possible home purchases.

Daniel Hershkowitz, a San Francisco real estate agent, said buying a home where the previous owner had remodelled it without the required city approvals can lead to greater financial risk for the subsequent homeowner. A good agent will encourage buyers to do their research to assess which rooms or extensions, he says, may be problematic. The first step is to compare the seller's disclosures with reports and tax records that revealed any permissions at the time of the previous sale, with the results of the inspector's checks.

To be even more cautious and concerned that a buyer who may find potential unauthorised work may be found, hire a local architect or licensed contractor to assess the potential risks and, if necessary, estimate the cost of making the home legal.

Shawn Ajdari, owner of AYA Homes, a home building and remodeling company, notes that unauthorised construction, especially illegal additions, is quite common in San Francisco. In a normal housing market, buyers would refuse to buy a home without a permit, but in San Francisco's overheated market, some buyers are willing to do so. He recommends that home buyers hire an expert, such as a contractor or structural engineer, to review suspected unauthorised work or poor quality construction to assess potential costs before making an offer to buy a home.