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Secondary suite (or accessory suite) is an urban planning term used mainly in North American English for an additional separate dwelling unit on a property that would normally accommodate only one dwelling unit. In British English the term "annexe" is used instead.
Relationship to main residence
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. Such a suite often is one of the following types:
- A suite above a rear detached garage (a "garage apartment"),
- A suite above the main floor of a single-detached dwelling,
- A suite below the main floor of a single-detached dwelling (a "basement suite").
- A suite attached to a single-detached dwelling at grade, or
- A suite detached from the principal dwelling (a "garden suite" or "guesthouse").
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A mother-in-law house is a form of guest accommodation that may be either completely detached (a guesthouse) or attached to a primary residence. An attached mother-in-law house ("granny annexe" in British English) often involves some spaces shared in common with an attached primary dwelling. It may, for instance, have a separate bedroom, bathroom, and kitchen, while sharing a living room and laundry facilities.
Mother-in-law houses are sometimes abbreviated as MIL in internet or newspaper listings.
A mother-in-law apartment is a small apartment accessory to a primary residence. Alternative names include "granny flat", "granny suite","in-law suite", and "accessory apartment", the first being used primarily in Australia, Britain, Ireland and New Zealand, where it is the most familiar of these terms, but also in parts of the United States. Such apartments are frequently used to accommodate an elderly relative who is incapable of independent living, but is not ready for a nursing home environment or other similar facility.
The apartment may or may not have a communicating door to the main house, but virtually always has a separate entrance and is usually not part of the original design. Many are located above the garage of the main house or as a separate building in the rear lawn.
In many North American municipalities, secondary suites are illegal because they do not conform to the zoning or land use district the property is in, they have been developed without the proper permits, or they do not meet the local building code. However, some localities only prohibit the renting out of secondary suites, and allow occupation by a relative or guest, leading to the use of the term "mother-in-law" house or apartment. Local jurisdictions may have rules regarding allowing certain relatives to live there and rules about what, if any, rent may be charged.
Dual occupancy is sometimes used to refer to the development of two dwellings on one allotment of land. They may be either attached (semi-detached) or detached. The term is common in Australia. Colloquially, these are most commonly known as a 'granny flat' in Australia. 
A Dual Occupancy in Victoria,Australia is a means to add value to property through a subdivision of one lot into two. It is that subdivision that makes dual occupancy an attractive proposition for new property investors and developers.
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:
- Southern Areas of Canada: $24,000/unit
- Northern areas of Canada: $28,000/unit
- Far northern areas: $36,000/unit
A 25% supplement in assistance is available in remote areas.
The Housing Policy Branch of British Columbia's Ministry of Community, Aboriginal and Women’s Services published a guide for local governments to implement secondary suite programs called 'Secondary Suites: A Guide For Local Governments'. The current issue is dated September 2005. The intent of the guide is to "help local governments develop and implement secondary suite programs". It also highlights good secondary suite practices as well as providing practical information to "elected officials, planners, community groups, homeowners, developers, and others interested in secondary suites".
In Norway, particularly in the bigger cities, it is quite common to build separate adjoined smaller flats for renting out. The owner of the main flat will rent out the smaller adjoined flats.
In Honolulu, Hawaii, they are called "Ohana Dwelling Units" or "Ohana Housing". Ohana Dwellings in Hawaii were created in 1981 as a way to encourage the private sector to create more housing units (without government subsidy), preserve green fields (open space) and ease housing affordability.
- Planning & Land Management, Territory Planning Branch (2002). "Dual Occupancy Review" (PDF). Retrieved August 21, 2008
- "'granny flat'". Macquarie Pocket Dictionary (3rd ed.). Brisbane: John Wiley & Sons Australia. 1998.
- Residential Rehabilitation Assistance Program (RRAP) -- Secondary/Garden Suite
- City and County of Honolulu. "Revised Ordinances of Honolulu, (ROH) Section 21-8.20" (PDF). Land use Ordinance. Retrieved 15 August 2011.
- "Ohana Housing Program Evaluation". Honolulu: Office of Information and Complaint, Sept. 1984. Retrieved 15 August 2011.
- Lau, Questor (May 2014). "Black boxes and gray spaces: how illegal accessory dwellings find regulatory loopholes.". Retrieved 4 August 2016.
- "Ohana Zoning a 5 year review" (PDF). State of Hawaii Legislative Reference Bureau. Retrieved 22 January 2015.
- "Maui County Zoning Code, Section 19.35". Retrieved 15 August 2011.
- "City of Santa Cruz Accessory Dwelling Unit Development Program". Retrieved 29 October 2015.