Oregon Middle housing (ORS 197A.420): what it allows on a lot
Middle housing (ORS 197A.420) is Oregon law, and the official source is at oregonlegislature.gov.
A city of 25,000 or more, and every city and county inside the Portland Metro district, must allow duplexes, triplexes, fourplexes, cottage clusters and townhouses wherever detached homes are allowed; a city of 2,500 to 25,000 must allow a duplex on each such lot.
Whether it applies to one lot turns on where the lot is and what it is zoned; the city, county or state office named in the law decides. Mapped areas are approximate. This is a plain reading of public law with its source, not legal advice.
Free, no account. ParcelSheet shows the lot's mapped size, parcel lines and flood zone, and this law on the lot's Oregon sheet.
Other Oregon laws on a lot: Accessory dwellings (ORS 197.312), Statewide planning goals.