A Reagan administration proposal to increase from 25 to 30 percent the share of their income that poor people must pay as rent for subsidized housing was defeated on a straight party-line vote yesterday by the House housing subcommittee.

The adminstration's entire housing assistance program was voted down, 19 to 14, as the Democratic-controlled subcommittee stayed with the more expensive program drafted by the Democrats. Not only did the subcommittee insist on holding down the percentage of income that residents of public housing and other subsidized units must pay in rent, it also provided for adding 250,000 units to the assisted program next year, instead of 175,000 as President Reagan proposed.

About 210,000 units were added to the total of subsidized housing this year, and the full House, which is more conservative than the housing subcommittee, may vote to cut that back.

Rep. J. William Stanton of Ohio, senior Republican on the committee, urged adoption of the Reagan figures, saying the subcommittee could save itself embarrassment later when it almost certainly would be forced to cut back the Democratic program to meet budget figures.

Stanton said the Reagan figures would jive with budget cuts voted by the House last week, but that adoption of the Democrat's housing program would require a major rewriting of it later.

But Rep. Barney Frank (D-Mass.), a liberal who supports the bigger assistance program, said, "I don't believe in jumping off bridges until we have to."