Seattle’s single-family zones already include some apartment buildings and accessory units. Thousands, it turns out.

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Mayor Ed Murray last year said he wanted to allow more types of housing in the single-family zones that cover much of the city, and some homeowners loudly objected.

Then Murray backtracked, setting aside the proposal to permit duplexes, triplexes, stacked flats and other multi-family structures.

The density debate has continued. A ruling came Tuesday on a legal battle over legislation that would allow more and larger accessory units — backyard cottages and mother-in-law apartments — to be built in the zones.

Along the way, you may have noticed something peculiar: Seattle’s single-family zones already include some apartment buildings and accessory units.

Thousands, it turns out. Researchers at Sightline Institute published a map Monday showing all of the multi-family structures in the city’s single-family zones. The buildings occupy nearly 4,600 lots and contain more than 10,000 homes, according to the think tank, a proponent of more flexible zoning.

In single-family zones, duplexes alone number 2,275, according to Sightline.

How is that possible? Most of the structures in question were built before the neighborhoods they stand in were zoned for single-family homes. The city said no new apartment buildings would be permitted but allowed the existing buildings to remain.