Proposal:Accessory Dwelling Unit

Accessory Dwelling Unit
Proposal status:	Draft (under way)
Proposed by:	Aharvey
Tagging:	dwelling=accessory
Applies to:	, ,
Definition:	Used to specify that a building is an accessory dwelling unit (ADU), a secondary self-contained dwelling on the same property as a principal dwelling.
Statistics:
Draft started:	2025-03-13

Preface

According to Wikipedia, "a secondary suite (also known as a 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 [or garden] on the same property."

While known by many different names in different places, I'll adopt the US term "ADU" for simplicity within the proposal.

The intent is to allow regions to translate this term through existing presets translations, so local mappers can see their local term. In the US localisation this may be "Accessory Dwelling Unit" in the Australian localisation this may be "Granny Flat".

ADUs may or may not have their own address.

Proposal

This proposal aims to gain acceptance for how to tag an ADU, with the view that the existing building=* scheme should be used for the type of structure, and a new dwelling=accessory tag introduced to specify a detached dwelling as an "accessory" or "secondary", an ADU.

The existing building=annexe is problematic, firstly because it's a top level building tag, secondly because it's documentation does not reflect actual usage. Much of the actual use is for "outbuildings" with the documentation as an ADU being added later without actual changes in how the tag is applied.

Rationale

Taking a step back and reviewing existing tagging schemes for mapping dwellings. We have two main schemes in use.

• 1) The longstanding building=* scheme
  • building=residential - broad classification, should be replaced with a more detailed tag
    • building=apartments - multiple units within the building, with access via a common entrance/hallway
      • usually the apartment building will have a single address, and each unit within will have a unit/flat number
        • addr:housenumber=123 - the house number/street address of the building
        • addr:flats=1-12 - the building has addressable units/flats from numbers 1 through 12 within it
        • building:flats=12 - the building contains 12 units/flats within it
      • where the apartment building has multiple entrances with different units/flats behind each entrance
        • on the building=apartments way
          • addr:housenumber=123
          • building:flats=12
        • node on the building=apartments way
          • entrance=main
          • addr:flats=1-6
        • second node on the building=apartments way
          • entrance=main
          • addr:flats=7-12
    • building=house - broad classification, should be replaced with a more detailed tag
      • building=detached - detached, freestanding residential house usually for a single family
      • building=semidetached_house - two houses built touching each other with a common wall, often with a seamless roof. Sometimes mapped as one area covering both houses, or as two areas with a shared edge.
    • building=terrace - a row of houses built together with shared walls - sometimes mapped as the area covering all houses, or as multiple areas, one for each house within the terrace
• 2) The newer building=house + house=* scheme (while in some locales a "house" usually refers to a strictly detached/freestanding house and therefore implies a specific structure, in the following we use the term house with a meaning more similar to "home", which is simply a self-contained living area for a family irrespective of the structure)
  • house=detached - detached, freestanding residential house, same as building=detached
    • addr:housenumber=123 - the house number
  • house=semi-detached - always mapped as two areas with a shared edge
    • where each semi house has it's own house number
      • addr:housenumber=123 (first)
      • addr:housenumber=125 (second)
    • where each semi house has it's own house number
      • addr:housenumber=12a (first)
      • addr:housenumber=12b (second)
    • where each semi house has the same house number but has it's own unit number
      • first
        • addr:housenumber=123
        • addr:unit=1
      • second
        • addr:housenumber=123
        • addr:unit=2
  • house=terraced - always mapped as multiple areas with shared edges
    • where each terrace house within a terrace row has it's own house number
      • addr:housenumber=123 (first)
      • addr:housenumber=125 (second)
      • addr:housenumber=127 (third)
    • where each terrace house within a terrace row has it the same house number but each has it's own flat number
      • first
        • addr:housenumber=123
        • addr:unit=1
      • second
        • addr:housenumber=123
        • addr:unit=2
      • third
        • addr:housenumber=123
        • addr:unit=3

Consider the different forms ADUs take in this graphic:

Tagging

• Detached Unit
  • Using first scheme
    • building=detached + dwelling=accessory
  • Using second scheme
    • building=house + house=detached + dwelling=accessory
• Attached unit
  • Only possible with second scheme
    • building=house + house=semi-detached + dwelling=accessory
    • the principle building may still be tagged as detached, since having an attached ADU may not be significant enough to mark the principle building as also semi-detached.
• Interior Upper Level Unit
  • no suggested tagging, however you may use building=detached + building:flats=2 to say there are two self contained flats within the building.
• Interior Lower Level Unit
  • no suggested tagging, however you may use building=detached + building:flats=2 to say there are two self contained flats within the building.
  • alternative may be possible with indoor mapping or building part, but has not been explored
• Above Garage Unit
  • no suggested tagging, however you may use building=garage + building:flats=1 to say there is one flat within the garage.
  • possibly with indoor mapping or building part, but has not been explored
• Garage conversion
  • Detached Garage Conversion
    • building=garage + building:use=house + house=detached + dwelling=accessory
  • Attached Garage Conversion
    • building=garage + building:use=house + house=semi-detached + dwelling=accessory

Further Comments

The main property of an ADU is they are on the same property (in some jurisdictions this may be a property parcel, property lot, or cadastral parcel/lot) as another dwelling. Frequently a fence will separate properties, whereas an ADU may not have a fence separating it from the principal dwelling.

In some circumstances it may be obvious on the ground that a dwelling is an ADU, it will appear as the odd one out compared to other dwelling on the properties, however other times it may be unclear if it is an ADU or considered as it's own principal dwelling, while it may formally be classified as principal/secondary based on cadastral data, planning documents or development approvals, such information is not always available in public or for use in OSM, and goes against the "Map what's on the ground" and potentially aspects of "Don't map local legislation if not bound to specific objects".

Further some may argue that the distinction between principle and secondary dwellings to too reliant on legislation and can be argued that it doesn't follow the core principles of mapping what's on the ground. Similar arguments are made about mapping property parcels.

Rendering

Map styles may choose to use a lighter colour for ADUs.

External discussions

• Talk:Tag:building=annexe
• https://github.com/openstreetmap/id-tagging-schema/pull/1422

Comments

Please comment on the discussion page.