THE 20-YEAR vision for a sustainable Washington that District Mayor Vincent C. Gray (D) released this week reads like a Christmas list: Some of the things sound doable — but how much is that pony going to cost?

There’s some irony in the mayor’s plan, given that Mr. Gray benefited during his 2010 campaign from criticisms that then-Mayor Adrian M. Fenty (D) spent too much on bike lanes and other green improvements. Now Mr. Gray is so enthusiastic about bike lanes , he wants 80 miles of them. And that’s among the least ambitious of his goals. He wants all new buildings to produce at least as much energy as they consume — if not more — and all existing ones to cut their power and water use in half. He wants the District to produce no net waste, through city-sponsored composting and reuse. He would quintuple the number of “green jobs” and cut carbon emissions 50 percent.

It is, in many ways, an appealing picture, and there are some things he can and should do soon, such as adding bike lanes, replacing street lights with more efficient ones, updating carbon goals for the electricity sector and cleaning up the District code to encourage smart electricity meters, so people know and can easily adjust how much energy they’re consuming. Some of the big-ticket items, such as preventing more urban runoff from fouling waterways, are worth pursuing once the costs are clearer.

But particularly without a sense of how much some of the big proposals would cost — and who would pay — much of the plan comes across as wishful thinking, and with contradictory wishes at that.

What’s more important, for example: achieving maximum environmental benefits at the lowest cost or creating the maximum number of “green jobs”? Cutting the most carbon emissions for the least investment or ensuring that carbon cuts come from solar power?

And what’s the mayor’s preference: making sure that the city’s growing population — up 250,000 over the next two decades, Mr. Gray predicts — has affordable places to live or demanding that all new buildings be “energy-neutral”?

Robust building codes, for example, make sense, particularly when they can be shown to save money over a reasonable amount of time. But if extra-stringent regulations on buildings cost developers and homeowners lots more, building up Washington would be harder. If costs instead fall on residents in the form of higher taxes or fees, Washington would become a less attractive place to live or to invest.

The mayor’s sustainability plan notes that it is only “a picture of what is possible” and that a serious implementation strategy will come in the next six months. There’s a lot of work still to do.