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.All this for $700 a month?  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.