Efforts to promote affordable housing in Boston are starting to sound like potato chip marketing, boasting “40% less fat” for a bag of chips that’s actually 40% smaller.  The Boston Redevelopment Authority is now authorizing apartment units as small as 450 square feet within one mile of public transit stops.  The problem, according to Boston’s chief planner, is that rents for these micro units are reaching $2,300 a month, or over $5.00 per square foot, and finding a roommate to share the rent isn’t a realistic option.  The planner mentions the need for a “better mechanism” to keep prices affordable, but beyond politically unpalatable rent control, it’s hard to imagine what that might be.  And while there may be a market for this type of housing in Boston, micro apartments don’t do anything about the high cost of that next step many young professionals want to take – the purchase of a condominium or a single-family home.  Perhaps it would be wise to design these micro units so they can easily be combined into larger spaces.