A key component missing here (and in Grant's rhetoric) is *what* that affordability level is. Seattle's MHA program is targeted at folks making 60% Area Median Income (AMI), while San Francisco goes up to 90% (and in New York City, it goes up to 150%). There is also the question of the "give back" required. In Seattle, it's 10 feet in building heights citywide, and converting 6% of SF zoning into low-rise zoning or RSL zoning. In San Francisco, it's a substantial tax break and use of LITC's. In NYC, it's double building heights and a 20-year property tax break. 

MFTE (12 year tax break) has a 20-25% requirement for 80% AMI homes, and when combined with MHA and Incentive Zoning, the actual % in place in Seattle is 27-36% below-market-rate unit production through the market, plus the homes built with the levy. It's a LOT more complicated, with statutory and constitutional issues, than just saying "25%!" Grant knew that in 2015 when he called for 5-10%. A shame he lies now.