Earlier this week Seattle moved to approve accessory dwelling units and also limit the size of McMansions:

While loosening restrictions on accessory units, the new rules will tighten restrictions on the construction of single-family houses by outlawing certain large homes, based on their floor area and lot size.

The aboveground living space for a single family in a new house will be limited to half the square footage of the home’s lot. For example, a new house on a 6,000-square-foot lot will be limited to 3,000 square feet of aboveground living space, not counting space devoted to an accessory unit.

O’Brien has said the “McMansion” ban will discourage people from replacing modest older houses with more expensive new houses and will encourage them to add accessory units….

The city’s most recent environmental analysis estimated the new rules would result in 4,430 accessory units built and 1,580 houses torn down over 10 years, versus 1,970 accessory units built and 2,030 houses razed under the status quo.

Three quick thoughts:

1. The restriction on floor size based on lot size is a common one. This not only keeps the overall size down but also can help keep the new home from crowding up against the edge of the lot. Fitting a large home on a small lot is a common issue with those opposed to teardown McMansions.
2. The story above says there is a restriction on aboveground square footage based on the lot. Does this mean some new homes will go underground instead (a la London)?
3. The numbers cited at the end of the article are interesting in a city of over 700,000 people. The “McMansion ban” would not appear to have much effect: over the course of ten years, roughly 25% fewer teardowns would occur according to projections. Even with the restrictions, it will still be an option for those with wealth who want a single-family home (as opposed to the new option of an accessory dwelling unit on the lot of someone else). If the city really wanted to go after McMansions, could they have done more?