A House-Senate conference committee reached agreement on a major housing and community development bill yesterday with one important exception: the amount of spending it would authorize.

Democratic conferees want a $15.3 billion measure, but Republicans said they will not sign the conference report unless the level is dropped to $15 billion.

Sen. Alan Cranston (D-Calif.), chairman of the Senate Banking, Housing and Urban Affairs Committee's housing subcommittee, said Democrats will agree to the lower figure if they are assured an override of the expected veto. The committee plans to meet Thursday or Friday to try to resolve the dispute.

"If the bill comes in at around $15 billion or $15.3 billion, it's too high from the administration's standpoint" and is a sure target for a veto, said Joseph R. Wright Jr., deputy director of the Office of Management and Budget. President Reagan's budget request for the Department of Housing and Urban Development for fiscal 1988 was $10.2 billion.

Final spending levels for individual programs were not set, but the House and Senate bills would hold spending for most housing and development activities at current levels, providing $3 billion for Community Development Action Grants and $225 million for Urban Development Action Grants, a program the administration has been trying to eliminate for several years.

Low- and moderate-income families displaced when their homes are demolished or converted to other uses because of development funded by the two federal programs would be given other affordable housing for 10 years, as well as money for moving expenses.

More than 900,000 units in privately owned buildings are in danger of being lost to the low-income housing stock over the next decade as regulatory restraints ensuring their use as low- and moderate-income housing expire, according to supporters of the measure.

Under the bill, owners would be permitted to pay off their federally insured loans and convert their units to market rate rentals or other uses only in accordance with a government-approved plan that would prevent tenant hardship or displacement when other affordable housing is unavailable.

The Senate and House authorization bills also contain about $7.5 billion for low-income housing assistance, $1.6 billion for public housing operating subsidies and about $2 billion for rural housing, enough for about 88,500 units.

The Federal Housing Administration's home mortgage insurance program would be permanently reauthorized, and a $100 billion limit would be set on mortgages the FHA can insure next year. Congress hopes to avert future shutdowns of the FHA insurance program.