INCLUDED IN the appropriations bills that President Reagan signed into law last week was authority to spend almost $10 billion on housing aid for low- and moderate-income families. Since the president has sought only a small fraction of that amount, accepting this new money marks a substantial retreat on his part. But the administration has, nonetheless, contributed to a substantial shift in the nation's approach to housing assistance.

The administration had sought to end direct federal involvement in construction of low-income housing except for units for the elderly and handicapped. Huge commitments from earlier years would have kept spending for housing aid at over $10 billion next year, but new help for low-income households would be limited to vouchers that would help pay rent on units meeting standards of quality.

In arguing for its position, the administration could point to the fact that the country is already obligated to future housing subsidies in the neighborhood of $250 billion over the next few decades. While the amount of substandard housing has dropped dramatically, direct construction programs have tended to produce unworkable concentrations of problem-prone families in public developments or privately developed units at exceedingly high cost. By giving some families vouchers with which to shop for housing, the administration hoped to encourage landlords to improve shoddy units and to open up a larger range of existing units to limited income families.

The appropriations bill would continue to increase the government's commitments to housing aid--though at a substantially slower rate than in the past--and, to a great extent, it buys the administration's approach of shifting money away from new construction. Most of the money would be spent under an existing program of rent subsidies that differs from the proposed voucher program in relatively minor ways. The remainder would be spent on new public housing, rehabilitating private housing and on an unspecified collection of voucher or construction programs.

The details of these programs remain to be hashed out in the housing authorization bill which has been held up in the Senate by conservative opposition to new commitments in the face of yawning deficits. By signing the appropriations bill the president has undercut this opposition, but there are still significant disagreements between the House and Senate on the total amount of new spending (the Senate wants less), on rents to be charged to subsidized tenants (the House wants to roll back recent increases) and on whether there should still be some money for private construction of rental housing. Nonetheless, both chambers have moved toward agreement that past approaches to low-income housing have yielded too little for their high cost and that future construction programs should focus on those special groups--the elderly, the handicapped and large families--that the regular housing market is least likely to serve.