The wealthy may have a whole other reason to dread President Joe Biden’s proposal to overhaul taxes on inheritances: They may have to dig through decades’ worth of documents to figure out what they owe Uncle Sam.

To help fund his American Families Plan, Biden is proposing higher taxes on capital gains and income for the wealthiest families.

He is also calling for the elimination of a decades-old loophole that allows individuals to inherit appreciated assets at market value with no taxes on the unrealized gain. This tactic is known as the “step-up in basis at death.”

Biden proposes ending this “basis step-up” for gains in excess of $1 million for single taxpayers – $2.5 million for couples – and ensuring that gains are taxed if the property isn’t donated to charity.

Coupled with a bid to raise long-term capital gains rates to 39.6% from 20% for households making over $1 million, wealthy heirs could be in for a pile of taxes.

But determining Uncle Sam’s cut could be difficult for certain assets. That’s because the basis – or the owner’s original investment in the asset – could be hard to track.

“How do you estimate the basis, especially when the person who had the best chance to answer that is deceased?” asked Ed Zollars, CPA and partner at Thomas, Zollars & Lynch in Phoenix.

Basis is important because the amount that you’ll pay in taxes when you sell an asset is based on the difference between the purchase price and the market value.

Without the “step-up in basis at death,” heirs would receive the asset with the decedent’s basis. This also means the heir could face a big tax bill on gains that have accumulated over decades.

This can pose a conundrum for complex assets, including real estate and small businesses, that rely on years’ worth of documents to determine the basis.

Owners of flow-through entities – limited liability companies, for instance – are subject to decreases in basis when they take distributions from their businesses. Meanwhile, capital that’s invested into the business increases basis.

“For a flow-through entity that’s been around for 45 years, in theory, I’d have to go through 45 years of tax returns,” said Brad Sprong, national tax leader of KPMG Private Enterprise. “Many times, records aren’t handily available, and obtaining transcripts from the IRS is hard, too.”

Real estate is another complication. Owners can defer capital gains on real estate by swapping one investment property for another in what’s known as a 1031 exchange.

Investors who perform these exchanges over many years and fail to keep proper records risk losing track of their basis over time, which can lead to complexity when they sell the property and try to determine the capital gains tax bill.

Zollars once had a client who sold property that was purchased in the 1970s and had gone through multiple 1031 exchanges. “They kept no real records,” he said. “That’s the manual research, going to the county assessor’s office to find some record of sales and purchases, which at best would be indirect.”

Even tracking basis for long-held publicly traded assets can get complicated.

Custodians, mutual funds and brokerages were only required to comply with federal basis tracking rules starting in 2011. Investors with holdings prior to that period can run into problems chasing basis if they switched brokerage firms or if they were in dividend reinvestment plans.

“Where it gets complicated is when you’re reinvesting all the time,” said Tim Steffen, CPA and advisor education senior consultant for Pimco. “Sometimes people forget they’re doing that. They forget they get all that extra basis and finding the records to report it can be a challenge.”

If you’re unable to determine the basis – and thus the magnitude of the taxes owed – it’s deemed to be zero.

Under Biden’s proposal, that would mean an heir would be subject to taxes on the entirety of the asset’s appreciation, minus the $1 million exclusion for single taxpayers ($2.5 million for couples).

Right now, when investors track down basis for long-held investments in the event of a sale and they come up short on documents, they may try making a good-faith estimate to determine the tax hit. Be warned: The IRS can contest your estimates and methodology.

Here are three steps to get ahead of a battle with the taxman.

Find your basis now and gather any supporting documents. When it comes to real estate and small businesses, valuation documents and paperwork that shows reinvestment into the property and improvements can also help you triangulate your basis.

Keep immaculate records, including statements that show when you’ve sold your holdings or when you’ve taken distributions from your partnerships.

Make the basis discussion part of your estate plan. In the same way your heirs should know where to locate your will, they should also have an idea of your basis in your assets. Share those documents with them ahead of time and save them the extra legwork of finding out what you originally paid for an asset.

Still can’t substantiate the basis? Consider charitable giving. If lawmakers do away with the step-up at death, assets with a basis that’s hard to ascertain might be good candidates for donation, said Steffen.

You can get a tax deduction based on the fair market value of the asset if you donate it to a qualifying charity. Think about it, but don’t act just yet.

“Wait for the details,” said Zollars. “Any proposed bill is just a proposed bill, but it’s not bad to get your basis together.”