Imagine that you own a house in Columbia neighborhood with an open back yard by the alley.

You just got a small inheritance from your grandmother and you have an idea — use the money to build a garage in back with an apartment on top. That would provide shelter for your car and some extra income from the rent.

But there’s a problem. Bellingham doesn’t allow garage apartments — traditionally called carriage houses — in single-family areas. And your neighbors worry about how big it would be and whether it would blend in well.

Now, pose the same situation but imagine that you live in a part of town with driveways instead of alleys. Would a carriage house work there?

About 100 city residents discussed the nuances of carriage houses and other types of alternative housing Wednesday at the second of four sessions of Bellingham’s Planning Academy II.

The workshops are for neighborhood activists, business people, builders, architects and other residents to discuss new types of housing that could help Bellingham absorb more people while treating its neighborhoods gently in the process.

Bellingham’s growth plan envisions 31,600 more people in the city by 2022, with a third of them settling in established neighborhoods, a third in highdensity urban villages and a third in new housing on the edge of town.

With an eye to figuring out where those people will live, folks at the Planning Academy discussed carriage houses and five other housing types for possible use in single-family neighborhoods:

Accessory dwelling unit (don’t you love land-use terms?) — a small house sharing a lot with another house. It’s usually built by the homeowner and often is rented.

Duplexes and triplexes — single- family housing that’s side-byside or stacked on a lot.

Courtyard housing — several houses that are on their own lots but share common vehicle access and courtyard space.

Houses on small lots — small houses on parcels of, say, 3,000 to 5,000 square feet. In Bellingham, single-family lots must be at least 5,000 square feet in most cases.

Cottages — a group of smaller houses clustered around shared open space and with shared parking.

New approaches to housing raise lots of questions: How will privacy be preserved? Will the new houses blend in with the neighborhood? What about parking and traffic?

Should new housing go in certain locations or be scattered through a neighborhood? Will some styles of housing work better in some neighborhoods than in others?

City officials hope the academy will produce a core of people familiar with a variety of housing options and with the questions that need to be answered to decide which types best fit Bellingham’s varied neighborhoods.

The crowd Wednesday night noisily discussed the six types of housing in clusters around 10 tables. At the end, each table’s captain came forward to summarize their comments, complaints and puzzlements.

Only one captain mentioned the issue of whether existing neighborhoods should absorb extra residents at all. That may have been the most surprising, and encouraging, moment of the evening.