Units in the size California allows as a second dwelling on a residential lot, from builders that state delivery to the state.
Permit rules are set by the city or county.

Cavco Homes · 900 sq ft · Starting price $65K.
Solitaire 16461A: 1-bedroom, 1-bath single-wide home with a 6 ft built-in porch; part of the Solitaire Singlewide series in the Ovation line

Cocoon Homes · 500 sq ft · Starting price $104K.
Two fully enclosed bedrooms, oversized living area, vaulted ceilings, full-size kitchen with multi-use island, full-sized bathtub or shower, lighted vanity.

Cocoon Homes · 400 sq ft · Starting price $89K.
Full-time living home with end cap kitchen, laundry capability, vaulted ceilings, and large windows.

Mint Tiny House Co. · 400 sq ft · Starting price $130K.
Newly built U.S.-certified 10 ft × 40 ft park model; features and finishes shown are included in this inventory home

Masaya Homes · 600 sq ft · Starting price $390K.
Site preparation and basic foundation with up to 40 feet of utility connections included · Up to 40 feet of utility connections included

Masaya Homes · 800 sq ft · Starting price $498K.
Site preparation and basic foundation with up to 40 feet of utility connections · Up to 40 feet of utility connections included