WHAT'S THIS ABOUT the Federal Home Loan Bank Board housing a disco? That's not the half of it. Besides the underground disco, which doubles as a cafeteria during the day, the board's sparkling new headquarters at 17th and G Streets NW has a skating rink in its courtyard, several shops on the ground floor and a French restaurant coming soon.

All this is quite a departure from the traditional, stonefaced, lock-up-at-5 federal building - and that's precisely what we like most about it. It's a cheerful example of the new federal policy of blending offices with commerce and community activities, to attract more people and keep an area alive after work and on weekends, when most federal buildings are dark, dead weights on their neighborhoods. Our only complaint about this open-door approach is that it came too late to save downtown from being saddled with the J. Edgar Hoover Building, among other forbidding hulks.

So we like the idea of combining financial offices with a disco - even one called "The Buck Stops Here." But we're not dancing in the streets about the way this project has been carried out. It started badly when the bank board insisted on the site across from the Old Executve Office Building and encouraged the General Services Administration to raze, peremptorily, some buildings that preservationists were trying to save. Then the bank board decided to manage the interior design and furnishings of its $51-million new home itself, and spent considerably more than GSA usually budgets for such things. Finally, the General Accounting Office and Sen. William Proxmire (D-Wis.) have charged that the board's leasing arrangements, especially with the restaurants, were more costly and less competitve than they should have been.

These problems in no way invalidate the open buildings concept. Nor is this a case in which GSA was the problem. The housekeeping agency, in fact, not only advanced a good policy, but did its part - construction of the building and renovation of the adjacent Winder building - rather well. The leasing mistakes were mainly the work of the previous management of the bank board. But GSA can certainly learn from those mistakes as it proceeds with an even more challenging project - the $18-million rejuvenation of the Old Post Office, now getting under way.