Prince William County's Comprehensive Plan Should Welcome Accessory Dwelling Units

Agency: Prince William County Planning Commission

I appreciate the opportunity to comment on the comprehensive plan of the Prince William County Planning Commission. The Mercatus Center at George Mason University is dedicated to bridging the gap between academic ideas and real-world problems and to advancing knowledge about the likely consequences of proposed regulation for private markets. Accordingly, this comment represents the views of no particular party or interest group.

The housing chapter of the draft of the comprehensive plan (dated January 25, 2022) contains a laudable and balanced overall vision for the county’s housing market. In particular, it recognizes the importance of accessory dwelling units to the diversification and attainability of the county’s housing stock. However, the plan hedges in its support for accessory dwellings, and experience has shown that even moderate-sounding government restrictions of accessory dwellings effectively stifle them.

The Best Time to Plan

An emerging scholarly consensus around planning, zoning, and housing is that people are most welcoming and open-minded about new and affordable housing during a broad planning process, and they are most hostile in case-by-case implementation. It has proven surprisingly easy to rezone entire cities, as in the case of Minneapolis, where rezoning a single block would have provoked vigorous local opposition. Piecemeal planning, which is the norm in many US locales unfortunately, is perceived as unfair by residents and rewards the loudest and best connected.

Although a comprehensive plan is not the place to spell out the narrowest details of a policy, it is an appropriate place to make specific policy commitments, such as committing to allowing accessory dwellings in all detached single-family homes or allowing garage conversions. The current draft language, which hedges with “where feasible” and commits only to “support” accessory dwelling construction, is at best a half step.

For accessory dwellings and other forms of housing, I encourage Prince William County to use the comprehensive planning process to form complete, specific plans for the zoning amendments, mapping changes, and other policy changes that will be necessary to implement the county’s laudable vision.

The Value of Accessory Dwellings

Accessory dwellings are rarely a major source of housing, but they support other sectors by creating flexibility at the edge of the housing market, easing booms and busts. For homeowners, they are on-site real estate investments that can be rented out during some seasons of life and used for one’s own purposes—a home office, for instance—during others. In pricey Vancouver, British Columbia, accessory dwellings are so crucial to enabling homeownership that they are commonly called “mortgage helpers.”

For renters, accessory dwellings are often the only source of small rental housing available in neighborhoods that otherwise offer only full-size houses. On a per-square-foot basis, accessory dwellings may be a bit less expensive than apartments. And some renters, especially friends and family members, arrange to pay below-market rent in exchange for performing household chores.

Getting the Details Right

As the county considers its accessory dwelling policy, it can be guided by best practices in other communities around the country. Please see the attached policy brief that I coauthored, “Ordinances at Work: Seven Communities That Welcome Accessory Dwelling Units.” My coauthor and I emphasize that to be successful at generating even a small number of accessory dwellings, ordinances need to be quite permissive.


Ordinances at Work: Seven Communities That Welcome Accessory Dwelling Units” (Mercatus Policy Brief)