When a property is being rehabilitated---rebuilt, whether substantially or not---the rehabilitation must include smoke detectors, in part to ensure the safety of the occupants, and in part to ensure that the property itself is protected.
The HUD website says all rehabilitation projects financed with Section 203(k) mortgage proceeds must have smoke detectors wherever people sleep in the home: "Each sleeping area must be provided with a minimum of one (1) approved, listed and labeled smoke detector installed adjacent to the sleeping area."
The FHA appraisal guidelines offer more specific instructions: "At least one operating UL-approved smoke detector, acceptable to the local building and fire codes, must be installed in each hallway adjacent to the bedrooms. On levels of the home that do not include bedrooms, at least one operating smoke detector must be centrally located."
Since the appraisal is part of the process of obtaining an FHA loan, this means that during the FHA appraisal process, someone will actually look at the smoke detectors. They have to be in the right places in the home, be operating correctly and be of a type accepted by the local building and fire code.
Like electrical codes across the country, fire codes in the United States are based on the National Fire Protection Association's Codes and Standards.
NFPA Code 72, the National Fire Alarm and Signaling Code, is the code dealing with smoke alarms. Section 1.1.1 deals with the "application, installation, location, performance, inspection, testing, and maintenance of fire alarm systems, fire warning equipment and emergency warning equipment, and their components."
Two types of smoke alarm technologies recognized most often are ionization-type smoke detectors and photoelectric smoke detectors. Ionization detectors have an element that uses a bridge of ionized air to allow a flow of electricity between two plates. Smoke disrupts this bridge of ionized air and triggers the alarm.
A photoelectric smoke detector uses a beam of light that's not quite aimed at a sensor. When smoke enters the smoke alarm, the smoke reflects the light onto the sensor, which triggers the smoke detector's alarm.
The NFPA recommends both types of detectors for optimum protection, but the FHA does not specify which kind is acceptable---only that a smoke detector "acceptable to the local building and fire codes" be used.