The Carter administration's new moves this week to rescue the slumping home-building industry are being regarded primarily as token gestures, unlikely to have much effect in stemming the current slide in housing starts.

Industry experts say the main impact of the proposals is likely to be more in helping builders sell off their costly inventories than in stimulating any substantial amount of new construction starts.

Even, there, the results are expected to be modest at best.

Administration officials told reporters Thursday that they estimate that 340,000 houses are currently sitting unsold in builders' inventories, and that the new proposed subsidies for builders could cover about 135,000 of those.

To qualify, the homes must have been built originally to FHA standards, or else must be able to pass FHA inspection.The new loans would allow the builders to pay off their construction loans. The maximum a builder can get from FHA for an unsold house would be $67,000.

In addition, officials said they intend to propose modifying the Section 235 mortgage-subsidy program to provide more -- although smaller -- subsidies to middle-income buyers. The current program is aimed at moderate-income families.

Currently, the mortgage subsidy program provides subsidized mortgages with interest rates as low as 4 percent to families whose income doesn't exceed 95 percent of the meian for their area -- an average of about $16,000 a year.

Depending upon the size of the family and home prices in the area, the maximum mortgage covered ranges from $32,000 to $44,000.

The new program would raise these income ceilings to perhaps 150 percent of median income, or $27,000 nationwide, and would provide buyers with smaller subsidies.

The interest rate would be 11 percent, with reductions for the first year to an effective rate of about 8 percent. The maximum mortgage would be raised to between $50,000 and $60,000.

White House officials said the program could stimulate construction of up to 100,000 houses, but industry officials, skeptical about the impact of the measures, say the results are apt to be more meager.