The 7 Most Expensive Estate Planning Mistakes High-Net-Worth Families Make

Estate planning done poorly is often worse than no estate planning at all. A poorly designed plan creates the illusion of protection while leaving significant exposure unaddressed — and it can be years or even decades before the problems surface. By then, the cost of fixing them often exceeds the cost of the original mistake. What follows is a detailed guide to the seven most expensive estate planning mistakes we see in high-net-worth family situations, why each one happens, and what the right approach looks like.

Mistake 1: Treating Estate Planning as a One-Time Event

Estate plans become stale. This is the single most pervasive failure mode in high-net-worth estate planning, and it is nearly universal.

The typical pattern: a family engages an estate attorney at a moment of trigger — a business sale, a large inheritance, the birth of a child, or a near-miss health scare. The attorney produces excellent documents. The family signs them, pays the bill, and files them away. Fifteen years later, the documents still reference a business that was sold, a trust that was never funded, and an exemption structure that no longer reflects current law.

Why estate plans go stale faster than most families realize: Tax law changes constantly. The estate and gift tax exemption alone has changed dramatically multiple times since 2000. A plan built around a $3.5 million exemption is not optimal for a $13.6 million one. Asset values change. A business worth $4 million when the plan was drafted may be worth $20 million today. Family circumstances change. Marriages, divorces, deaths, estrangements, disabilities, and new children all create planning implications the original documents did not anticipate.

The fix: estate plans should be reviewed at a minimum every three to five years, and immediately upon any significant change in tax law, asset values, family composition, or business structure.

Mistake 2: Not Using the Elevated Exemption Before 2026

The Tax Cuts and Jobs Act of 2017 raised the federal estate and gift tax exemption above $13 million per person. That exemption is scheduled to be cut roughly in half at midnight on December 31, 2025.

For high-net-worth families with estates above $7 million per person — the approximate post-sunset exemption — the difference between acting and not acting before that deadline is enormous. Consider a married couple with a combined estate of $25 million. Under current law, their combined exemption is approximately $27.98 million — enough to shelter the entire estate. After the sunset, their combined exemption drops to approximately $14 million, leaving $11 million exposed to 40 percent estate tax. The cost of inaction: approximately $4.4 million in avoidable estate tax.

The strategies available — gifts to irrevocable grantor trusts, SLATs, GRATs, intra-family sales — are well established and IRS-approved. The anti-clawback regulation issued in 2019 confirmed that gifts made using the elevated exemption before the sunset will not be taxed again at death even if the exemption has since decreased.

The mistake is not ignorance of these strategies. It is inertia. Families who know they need to act but keep waiting — for a better time, for more certainty, for the legislative picture to clarify — are making a decision by not deciding. The exemption does not wait.

Mistake 3: Funding Trusts Without Coordinating the Tax Reporting

This mistake is subtler than the previous two but arguably more dangerous because it is invisible until it is too late. The pattern: a family works with a skilled estate attorney to design and execute a sophisticated trust strategy. The documents are excellent. The transfer is completed. And then nobody coordinates the tax reporting.

The gift tax return (Form 709) must be filed for the year of the transfer. It must properly report the value of the gift, document the use of exemption, and include the qualified appraisal that supports the transfer value. A 709 filed without the required appraisal — or filed with an incorrect valuation — is not just a compliance problem. It is an open invitation for the IRS to challenge the transfer years or decades later, with no statute of limitations protection.

Grantor trust reporting on the 1040 must be consistent from year to year. When the trust eventually becomes a non-grantor trust, annual Form 1041 filings are required. Trusts that have been operating without a required 1041 create significant back-filing and penalty exposure.

The coordination failure typically occurs at the intersection of the legal and tax relationships. The estate attorney considers the work done once the documents are signed. The CPA is not involved until tax season, by which point the reporting window may have already closed. The fix: the CPA and estate attorney need to be in communication from the moment a trust strategy is being designed, not after it has been implemented.

Mistake 4: Ignoring the Income Tax Side of Estate Planning

Estate planning focused exclusively on reducing estate tax can inadvertently create massive income tax problems. This trade-off is real, it is common, and it is almost never modeled correctly by families working with advisors who specialize in only one side of the equation.

The core tension: assets transferred out of the estate during lifetime save estate tax but carry a carryover basis. Assets held until death receive a stepped-up basis to fair market value at death — which can eliminate unrealized capital gains, including decades of appreciation and depreciation recapture on real estate.

The numbers can be staggering. A real estate portfolio with a $10 million fair market value and a $1 million adjusted basis carries $9 million of unrealized gain. If transferred to an irrevocable trust during lifetime at a carryover basis and later sold, the combined tax cost could approach $2.7 million. If the same property had passed through the estate at death, the basis would step up to $10 million and all $9 million of gain would disappear.

The right answer varies by asset. High-basis assets with significant future appreciation are candidates for lifetime transfer. Low-basis assets with modest appreciation often benefit from the basis step-up strategy. Any estate planning engagement should include a full basis analysis of all major assets before any transfer decisions are made.

Mistake 5: Using the Wrong Trust Structure for the Asset

Trust design is not one-size-fits-all. Certain assets have specific trust eligibility requirements that, if violated, can produce catastrophic tax consequences.

S corporation stock is the most dangerous example. An S corporation may only have eligible shareholders — individuals, certain trusts, and estates. If an S corporation interest is transferred to a trust that does not qualify as an eligible S corporation shareholder, the S election terminates immediately and involuntarily. The corporation becomes a C corporation, all undistributed earnings become subject to double taxation, and the tax history of the company is permanently disrupted.

Trusts that qualify as eligible S corporation shareholders include qualified Subchapter S trusts (QSSTs), electing small business trusts (ESBTs), grantor trusts during the grantor's lifetime, and certain testamentary trusts during estate administration. Each type has specific requirements that must be met and maintained.

IRA and retirement account beneficiary designations interact with trust design in ways that significantly affect income tax treatment of inherited retirement assets. Under the SECURE Act and SECURE 2.0, most non-spousal beneficiaries must distribute the entire inherited IRA within 10 years. Certain conduit trusts can extend this window; accumulation trusts that do not meet the conduit trust requirements are subject to the 10-year rule regardless of the trust's terms.

Mistake 6: Not Coordinating the Advisory Team

Estate planning done in silos — attorney drafting, CPA filing returns, investment advisor managing assets — without active coordination routinely produces outcomes worse than what any single advisor would design alone. The problem is not that any individual advisor is incompetent. The problem is that the interactions among their separate pieces of advice produce unexpected and harmful results.

A representative example: the estate attorney designs a GRAT that transfers the client's S corporation interest. The GRAT is structured as a grantor trust, appropriate for income tax purposes. What the attorney did not know — because the CPA was not involved in the design — is that the GRAT as structured does not qualify as an eligible S corporation shareholder, terminating the S election the moment the transfer is completed.

Another example: the investment advisor recommends transferring the client's low-cost-basis stock portfolio to a grantor trust as part of 2026 planning. After the transfer, the family discovers that the portfolio's total unrealized gain is $8 million — the income tax cost of losing the basis step-up at death exceeds the estate tax savings from the transfer.

These failures are not rare. They are the norm when advisors work sequentially rather than simultaneously. For any significant planning engagement, the CPA, estate attorney, investment advisor, and other specialists should be in the same conversation — ideally literally, in a joint planning meeting — before any strategy is selected or any document is drafted.

Mistake 7: Procrastinating Until a Crisis

The most expensive estate planning mistakes are often made under time pressure — or not made at all because the family was waiting for the right moment and the right moment never came.

Crisis events that force last-minute planning include: a terminal diagnosis, a pending business sale, a divorce, the unexpected death of a spouse, or sudden awareness of the 2026 deadline in late November 2025. Each of these events creates urgency — and each also destroys planning options.

GRATs, installment sales, and other strategies that depend on the asset appreciating over time need that time to work. A GRAT funded in October 2025 with a two-year term provides two years of appreciation opportunity. The same GRAT funded in 2022 with a five-year term provides five years. Business valuations, trust drafting, and transfer execution take 16 to 20 weeks for a well-executed engagement.

Health matters for certain strategies. A grantor diagnosed with a terminal illness after funding a GRAT may not survive the GRAT term — pulling the assets back into the estate and undoing the planning.

The most powerful estate planning strategies compound over time. A business interest moved to a grantor trust today begins accumulating appreciation outside the estate immediately. Each year of additional appreciation that occurs inside the trust rather than in the taxable estate saves 40 cents on the dollar in future estate tax. Starting five years earlier — with the same strategy — produces five years of additional tax-free compounding.

The fix: start the planning conversation now. Not at the next business milestone. Not when the legislation clarifies. The cost of starting early is the cost of the planning engagement. The cost of waiting can be measured in millions of dollars of avoidable estate tax — and those dollars cannot be recovered.

A Framework for Getting This Right

The families that navigate estate planning most successfully share a few common characteristics. They treat estate planning as an ongoing process rather than a discrete project. They engage advisors who understand both the legal and the tax dimensions and who communicate with each other. They act during windows of opportunity rather than waiting for certainty that never fully arrives. And they build in enough time to do things properly.

For high-net-worth families reading this list and recognizing one or more of these mistakes in their own situation, the right next step is a diagnostic review — an honest assessment of where the current plan stands and what gaps need to be addressed. The goal of that review is a clear picture: current estate exposure, the cost of inaction, the strategies available, and the timeline required to execute them.