If you own or plan to invest in real estate, there are three key aspects of President Reagan's tax-simplification plan that deserve your special attention.

You ought to know that:

* Buried deep inside the 461-page proposal are significant, little-reported changes in capital-gains treatment for real estate. You may have read the good news that the maximum capital gains tax would be lowered to 17.5 percent. What you didn't read, though, was that your real estate investment probably won't qualify for favorable capital-gains treatment in the future.

* Proposed new restrictions on deductions of "investment interest" go far beyond what they did in the so-called "Treasury I" plan released last November. Not only would second homes be socked by these deduction cutbacks, as proposed by the Treasury in 1984, but now investments in public and private partnerships that build and finance a major portion of the nation's residential rental and commercial real estate would be roped into the interest-limitation trap as well.

* "Effective dates," "grandfathering" and "transition" provisions could be the true tax-reform ballgame for millions of Americans who own or invest in real estate. The Jan. 1, 1986, effective dates attached to large numbers of proposed tax changes in the Reagan plan are almost meaningless because the package is unlikely to be enacted before 1986. The effective dates and transition provisions that could affect you the most haven't been written yet -- and won't be until this fall at the earliest.

The change in capital-gains treatment -- never mentioned by the president in any of his pro-reform speeches -- could come as a shock to tens of thousands of small-scale investors in rental homes, condos and other real property.

Essentially the revised plan would chop the top federal tax rate applicable to investments that have produced capital gains, but eliminate depreciable investment real estate (and other depreciable assets) from eligibility for capital-gains tax treatment.

To put the change in concrete terms, consider this example. Under current law, let's say you and your spouse buy a small house in your community to hold for rental and appreciation. Let's say its original cost for tax purposes was $70,000, and you took the normal tax write-offs for expenses and depreciation. After renting it out for five years, you decide to sell it to a home buyer for $100,000, giving you a nice profit.

Your profit under current law would qualify for capital-gains treatment. You could exclude 60 percent of the gain; the balance would be subject to taxation at the rate you pay on your ordinary income.

The president's proposal would alter this dramatically. It would require that you take depreciation on future investments over a 28-year period (up from 18 years currently). It would index your initial tax cost, or "basis" in the house, to a federal inflation measure. And it would deny capital-gains treatment for any part of your rental property (except the land underneath it).

In short, the bulk of your investment profits that are attributable to the increased resale value of the house itself would be taxed at your full, ordinary rate -- which could well be 35 percent.

The Reagan tax plan's revised treatment of "investment interest" is another potential shocker. Not only does it seek to limit total tax deductions on money borrowed for "consumer" and other nonbusiness purposes such as mortgages on second homes, car loans, credit charges and the like, it also includes interest deductions generated by real estate (and other) limited-partnership investments.

The rule proposed is this: Taxpayers could take deductions for the interest expenses of cars, second homes, boats, real estate limited partnerships, college tuitions, etc., only to the extent that the write-off didn't exceed the total of "net investment income" plus $5,000.

Add up your current interest deductions using this formula and see how you fare. If you're like many people involved in small- or medium-scale real estate -- through passive partnership investments or a vacation home at the beach -- you'll probably feel the pinch.