IF MAYOR Vincent C. Gray (D) has his way, the District will transform over the next two decades from an asphalt jungle to a city, with 250,000 additional residents, that is literally and figuratively green. According to a sustainability plan the mayor released Wednesday, by 2032 the District will use 50 percent less energy, emit 50 percent fewer greenhouse gases and produce 15 percent less waste. Roofs will have more plants, and so will sidewalks; the city will have planted more than 150,000 trees. High-efficiency bulbs will illuminate streets and miles of new bike paths. Metro will carry more passengers, and streetcars will ding their way across town.

Mr. Gray deserves credit for producing an ambitious development vision that recognizes the importance of managing this community’s many environmental impacts, even as government deals with other tough problems such as enduring pockets of poverty. Many but not all of the ideas he proposes make sense. There is a free-lunch thinking in the document, with true costs and trade-offs not addressed, that other city leaders should treat warily.

First, the good. Mr. Gray would begin immediately with such sensible steps as tracking the city’s energy use, expanding Capital Bikeshare, building more parks, planting 8,600 trees a year and gauging the District’s vulnerability to the potential impacts of climate change.

Perhaps the most promising short-term proposal is to revamp the web of municipal regulations that discourage more people from living closer together and near public transportation. He would also reform other rules that make it difficult to do business in Washington. Property owners, for example, would be able to convert basements or over-garage space into livable quarters with less hassle, and those on transit corridors would be able to build up. Longer-term policies — such as requiring a certain amount of the city’s electricity to come from clean sources or further investment in public transit — would complement these efforts. So might reassessing building codes to demand energy savings, since structures account for three-quarters of the city’s carbon footprint.

But sustainability doesn’t always come cheap, and that’s especially true when government tries to get too involved — for example, in deciding where the city’s electricity comes from. Mr. Gray’s idea to use municipal funds to finance a nearby wind farm, for example, is wasteful and unnecessary. The city already requires an increasing percentage of its power to come from renewable sources; it should allow private enterprises to figure out how to meet that requirement as efficiently as possible. And the city should not invest much in orchards or other efforts to source food locally.

Mr. Gray’s plan does not admit that there might be tension among its many goals. What’s more important: quintupling the number of green jobs or meeting low greenhouse-emissions targets cheaply and on time? Keeping housing costs down for a growing population or demanding that buildings in the District are net-zero-energy users? Who gets scarce city funds: Metro or a new small-business loans program? The District can’t do it all.

The mayor has put dozens of ideas on the table for discussion. Now he and the D.C. Council should set their priorities.