A secondary suite (also known as an accessory dwelling unit (ADU), in-law apartment, granny flat, granny annex or garden suite) is a self-contained apartment, cottage, or small residential unit that is located on a property that has a separate main, single-family home, duplex, or other residential unit. In some cases, the ADU or in-law is attached to the principal dwelling or is an entirely separate unit, located above a garage, across a carport, or in the backyard on the same property. Reasons for wanting to add a secondary suite to a property may be to receive additional income, provide social and personal support to a family member, or obtain greater security.

Naming conventions vary by time-period and location but secondary suites can also be referred to as an accessory dwelling unit (ADU), mother-in-law suite, granny flat, coach house, laneway house, Ohana dwelling unit, granny annex, granny suite, in-law suite, and accessory apartment. The prevalence of secondary suites is also dependent on time and location with varying rates depending on the country, state, or city. Furthermore, regulations on secondary suites can vary widely in different jurisdictions with some allowing them with limited regulation while others ban them entirely through zoning, limit who may live in the units (for example, family members only), or regulate if units can be rented.

A secondary suite is considered "secondary" or "accessory" to the primary residence on the parcel. It normally has its own entrance, kitchen, bathroom and living area. There are three main types of accessory units: interior, interior with modification, and detached. Examples include:

Linked properties cannot easily be sold separately. In case of shared ownership each party may require permission from the other party to make changes to the building.

In Australia, the term 'granny flat' is often used for a secondary dwelling on a property. The land is not subdivided with construction requiring approval from the council or relevant authority. The approval processes vary between States and Territories, and between councils. This is different from a dual occupancy, where two primary dwellings are developed on one allotment of land, being either attached, semi-detached or detached. In 2018, New South Wales led the construction of new granny flats while Victoria had the fewest number of new granny flats constructed. In 2019, the federal government launched a study concerning prefabricated buildings and smaller homes citing affordable housing, extra space for family members, and support for the construction industry as reasons for the study. The government set aside $2 million for the initial study and then plans to set up an innovation lab to help manufacturers design prefabricated buildings.

Secondary suites have existed in Canada since the 19th century where they took the form of coach houses, servant houses, stables converted to permanent apartments, and small apartments for young people within large houses. Secondary suites became increasingly popular during the economic crisis of 1929 and the housing shortage following WWII. During this period the Canadian government actively supported the creation of secondary suites. However, suburbanization and zoning changes in the 1950s and 60s led to a decrease in secondary suites in Canada. More recently, secondary suites are increasing in popularity and many municipalities are reexamining their regulations to support secondary suites.

The Canada Mortgage and Housing Corporation provides a financial assistance program to help Canadians create affordable housing for low-income seniors and adults with a disability within a secondary suite. The program is called the Residential Rehabilitation Assistance Program (RRAP) -- Secondary/Garden Suite. The maximum fully forgivable loan depends on the location of the property:

A 25% supplement in assistance is available in remote areas.