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C. Authorization of ADUs by Zoning District In the absence of state legislation addressing the issue, communities have wide discretion in permitting ADUs in many types of residential zoning districts. The merits of locating ADUs in the major types of residential zones is discussed below. As a general principle, in communities with high rents and home prices relative to incomes, the governing body should allow ADUs in the full range of zones where residences are authorized. Different zones and their suitability forADUs are discussed below. Mixed -Use Zones: In the last few decades, governments and planning advocacy groups (including AARP) have recognized the many adverse consequences of strict single -use zoning. Across the country, zoning has been reformed to allow a greater mixture of uses along with residential uses, such as institutional uses, professional services and retail commercial uses. Because of the success over the last century in reducing the pollution and noise impacts from many types of urban land uses, some communities have gone further and allowed residential uses intermingled within a wide range of nonretail commercial and light industrial zones. ADUs may not be appropriate on a variety of lots in these mixed -use zones, but they make sense on lots that are the site of a detached single-family dwelling. Multifamily Zones: These zones are distinguished by apartments or condominiums with multiple dwellings on the some lot, typically in multiunit and/or multistory structures. In recent years some cities with high housing costs have approved or are considering the authorization ofADUs on lots with multifamily structures. California requires jurisdictions to allow new ADU units to be created out of existing parts of multifamily buildings if those parts are not currently used as livable space, such as storage rooms, garages, or basements or through an addition to the building.'-' In May 2020 the Chicago City Council considered a draftADU ordinance that would allow new ADUs equal in number to 33% of the existing units in a multifamily structure on the lot. Town House Zones: These zones contain single-family dwelling units that have common walls but are not atop one another, typically one dwelling per lot. Siting ADUs in these zones can have its challenges, given building orientation and lot coverage. On the other hand, Washington, D.C., is an example of a city where many historic townhouses included an "English basement" on the lowest floors of the building. Ordinances addressing the creation ofADUs in these districts will need to provide more flexibility regarding both siting requirements and some building code standards (flexibility that does not compromise health and safety). Single -Family Zones: These zones contain one single-family dwelling unit per lot and provide the greatest opportunities for siting all types ofADUs. Some jurisdictions also allow clusters of small single-family homes, each on their own small lot or as condominium units with common space. Single-family zones also include detached single-family homes on their own lot and can be treated the some way as those homes are treated in single-family zones. Even in these single-family zones, however, neighbors' concerns about property values, aesthetics and "neighborhood character" have often caused communities to ban detached ADUs or to allow them only on larger lots. Perversely, this can mean that ADUs are prohibited in single-family zones with large lots and bigger houses, where they can be more easily sited as detached units or created by remodeling existing space, but allowed on small lots where this is more challenging. This kind of policy choice reinforces rather than reduces the impact of exclusionary zoning. For reasons of equity and to realize the benefits described in the statement of purpose and intent, ADUs should be authorized in all single-family residential zones. In adapting the model provisions to a local zoning ordinance, a community will substitute its zoning district 33 1 HARP - Accessory Dwelling Units: Model State Act and Local Ordinance