FOR TODAY'S EXAMPLE of good intentions gone awry, consider the housing aid that the federal government has just reserved for big cities. The District of Columbia is supposed to get $3.5 million or more, which could help many lower-income families. But it turns out that part of the money, perhaps one-fourth to one-third of it, will probably be unusable.That's because it is meant for public housing, which cannot be built here now. Why not? Because the federally set cost limits are too low. The city could benefit from another program that subsidizes new and rehabilitated rental housing. But - you guessed it - the District is not scheduled to get a cent for that this year.

How did things get so fouled up? Nobody meant to deal the District out. The trouble is that Congress and HUD have tried too hard to deal everybody in. Various lawmakers were worried about the housing needs of Indians, elderly people and the rural poor, so some aid were reserved for each of those groups. The congressional champions of public housing ordered HUD to spend a certain amount for that. The advocates of state housing-finance agencies directed that lots of rental aid be spent through those channels. And after every camp in Congress had had its say, HUD officials set aside a few more sums for special programs.

By the time all this reserving and earmaking and setting-aside was done, most of this year's $1-billion-plus in housing aid was tagged for some specific kind of use. In national perspective the results might well look reasonable. But when each category is sliced thin and parceled out across the country, someone is bound to get pieces that just don't fit. And that's what happened to the District: There is simply not enough usable aid for lower-income families who are not Indians, not elderly, not rural, and who don't live in a state with a housing-finance agency.

Local officials are asking HUD for special aid. That may be the only remedy this year. For the future, though, the answer surely is not to create still more categories for the District and every other special case that comes along. Instead of going further down that road, Congress and HUD ought to back off and give local governments more room to tailor programs to suit their needs. All of the abstract equity and interest-balancing in the world means little, after all, if it doesn't translate into real housing for people who need help.