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If you haven't gotten the scary email about the 3.8% real estate tax that's about to take effect, just wait. You probably will see it as the tax's effective date draws near.

Various versions of this email have been frightening and angering people since 2010, when the health care legislation that contains the tax was approved. Unfortunately, these emails get most of the facts wrong. But then, so do some of the tax's defenders.

Here's the scoop:

● It's not true that the tax, scheduled to start Jan. 1, will affect all home sales, or even most home sales. It won't cost you $3,800 in taxes to sell a $100,000 home, or $15,200 if you sell a $400,000 home, as the emails claim. "Some of the emails treat it as if it were a tax on the total sales price," said attorney Mark Luscombe, a CPA and principal federal tax analyst for tax research firm CCH. "It's not a sales tax."

● It's not true that the National Association of Realtors is working to get the tax repealed. In fact, the association has been trying to counteract what a spokeswoman called "grossly inaccurate" rumors about the levy.

● But it's also not true that the tax will affect only the very rich, as some pundits have said. In some circumstances, it could affect people who wouldn't normally be considered wealthy.

Image: Liz Weston

Liz Weston

To find out if the tax could affect you, or anyone you know, read on.

The tax is designed to raise an estimated $210 billion to fund Medicare and health care reform. It's a tax on so-called "unearned" income, including investments, rental income and home sale profits over a certain exemption amount ($250,000 for singles, $500,000 for married couples).

All this unearned income is already subject to ordinary income taxes or capital gains taxes. The new 3.8% tax would be added only for some individuals with incomes above $200,000 or some married couples with incomes more than $250,000, Luscombe said.

This is complicated stuff, which is part of why it's so easy to misunderstand. So follow along with this illustration: Let's say that Don and Donna bought a home in Southern California in 1965 for $50,000. Today, the home is worth $650,000. If the couple sold the house, their profit would be $600,000 (for now, we'll ignore the costs of selling and home improvements, which typically would reduce that number).

The first $500,000 of that profit would be tax-free, thanks to a federal law that went into effect May 6, 1997. The remaining $100,000 would be subject to a capital gains tax rate of up to 20% (the maximum capital gains tax rate is currently 15%, but that's scheduled to rise in 2013). It might also be subject to the 3.8% tax.

The next step is to add the $100,000 profit (or "net investment income") to Don and Donna's adjusted gross income. If the profit pushes their AGI over $250,000, the new tax would apply to either:

  • The net investment income amount -- the profit over the exemption limit of $500,000 ($250,000 for singles).
  • The amount that their AGI exceeds $250,000 ($200,000 for singles) in income

Don and Donna would owe tax on whichever of those two options is less.

Head spinning yet? Here's how it would work in our example.

If the couple's AGI was $50,000 without the home sale and $150,000 with it, they wouldn't be subject to the 3.8% tax. (They'd still have to pay the capital gains tax, however, which would be $20,000 on their $100,000 of profit above the exemption limit.)

If, on the other hand, their AGI was $200,000 and the remaining home sale profit pushed it to $300,000, they (or their tax software or their accountant) would compare the amount more than $250,000 ($50,000) with the amount of investment income ($100,000) and pay the 3.8% tax on the lower of those two amounts. So they would pay an additional $1,900 (3.8% of $50,000).

Are there circumstances when the tax could affect people who aren't big earners? You bet. If the home profit is large enough, it could push anyone into a bracket where that person might face the tax.

Let's assume now that Don and Donna's house sold for $900,000. After deducting the home's cost (and once again ignoring the costs of selling and improvements), their profit is $850,000. Subtract their $500,000 exemption, and you're left with $350,000. That's enough to trigger the tax, even if their AGI before the home sale was zero. Then they'd owe an additional $3,800 (3.8% of $100,000, the amount of net investment income over the $250,000 AGI limit), on top of capital gains taxes on the $350,000.

It's safe to say that the vast majority of home sellers won't pay this new tax, at least at this point. In July 2012, the median sales price for existing homes was $181,500, according to the National Association of Realtors. That means half of homes sold for less. Homes that sold for more than $500,000 accounted for just 11% of home sales.

But it's not reasonable to assume that all of those sales would have faced the tax, since some of the homes might have been sold at a loss and others would have less than the required appreciation to trigger the tax. As I noted earlier, the costs to sell a home, and improvements made to the house over the years, can shrink the potentially taxable profit on a home sale, sometimes dramatically.

Congress, however, hasn't adjusted the $250,000/$500,000 home sale exemption amount for inflation. So conceivably, more folks could face the tax in the future, especially if they live in high-cost areas or have a lot of appreciation in their homes.

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If you think you, or your folks, could be subject to this new tax, it might be smart to sit down with a tax professional to talk about alternatives. Closing a home sale before Dec. 31 -- or selling investments that could trigger the tax -- could save some money. Then again, such sales might not be prudent. For example, if your parents bequeath their home to you at their death, rather than selling it now, all the gains during their lifetimes would be essentially tax-free -- you may not owe any income or capital gains tax if you sell soon after you inherit (although their estates may owe taxes, if those estates are large enough).

In other words, don't let the tax tail wag the investment dog: Get good advice, and make sure any sales fit in with a long-term financial strategy.

Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy" (find it on Bing). Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Join the conversation and send in your financial questions on Liz Weston's Facebook fan page.