Congress is leaving behind some noteworthy tax gifts for real estate and home improvement financing this year.

And even the normally penny-pinching, uncompromising Treasury Department offered its own holiday cheer to property owners concerned about the controversial "family rental tax" and office-in-the-home proposals of the Internal Revenue Service.

The Treasury said it won't adopt the IRS's rules in final form until next July -- leaving Congress plenty of time to rewrite the law.

In sheer dollar terms, the biggest last-minute tax bonanza for housing came in legislation designed to encourage hundreds of city and county governments around the United States to float new bond issues for the benefit of first-time home buyers.

The new law allows for issuance of $10 billion or more per year nationwide of tax-exempt housing bonds by city governments, counties, small towns and states.

The funds raised from the bond issues can be used to provide:

Cut-rate mortgages to first-time buyers of houses with price tags close to the average price for dwellings sold in the local market. Since first-time buyers tend to purchase modestly priced units anyway -- town houses, condominiums or smaller suburban tract homes -- Congress' price limitations should pose no problems.

Home fix-up loans of up to $15,000 with below-market interest rates, regardless of the market value of the house being repaired or the income of the owner.

Energy conservation improvement loans of up to $15,000 for homeowners -- again with discount interest rates and without regard to property value or family income.

The actual interest rates paid by home buyers or property fixer-uppers will depend on the state of the money market in 1981 and beyond, as well as the characteristics of the bonds floated by local governments.

Although the new law puts restrictions on local governments' use of bond financing -- ruling out subsidized mortgages for higher-income families -- its net effect is to flash a green light for more cut-rate home loan programs.

One of the sleeper provisions in the new law undoubtedly will be in the area of energy conservation loans. Dozens of cities and counties in more than 25 states -- including Illinois, Louisiana, New Jersey, California, Texas, Colorado, Maryland, New York and Florida -- have issued mortgage bonds in the past two years.

But no city has yet put together a tax-subsidized bond issue aimed at financing energy-conserving home improvements for local residents, such as heavy-duty insulation, thermal glazing, and passive and active solar installations.

The "family rental tax" and office-in-the-home brouhahas -- chronicled in this column last August -- now seem headed for early resolution. The rental tax issue concerns the IRS's attempt to issue regulations denying normal tax deductions to homeowners who rent property to relatives -- whether a principal residence, a vacation home, mobile home or boat -- at market rates.

The IRS proposals last summer triggered a massive protest from real-estate owners to the Treasury, and an appropriations freeze by Congress. rIn a last-minute, conciliatory gesture before Congress adjourned, Treasury, sent a letter to Sen. William Armstrong (R-Colo.) pledging to take no action on the two tax issues until mid-1981 at the earliest.