The easiest solution is to give owners the legal right to improve their property. Right now, improvements such as putting an accessory dwelling unit in the rear of a lot are illegal everywhere. Changing that zoning law doubles the land available for development overnight without affecting the "character" of the neighborhood, forcing anyone to do anything, or hurting property values.

There are other solutions, such as legalizing the construction of townhomes and condos up to 3 stories on any residential plot; expanding the definition of "residential" to include coffee shops, restaurants, small stores, daycares, etc.; and changing taxes to disincentivize vacant space or large lawns in the city center. These all have pros and cons and various levels of political will, but the ADU solution is easy, cheap, and has no valid NIMBY arguments because it isn't something they would see in the first place.