10 Estate Planning Mistakes That Cost High-Net-Worth Families Millions

At $5M+ net worth, estate planning mistakes don't just create paperwork hassles — they create six- and seven-figure tax bills, contested distributions, and lost exemptions that can never be recovered. Below are the ten mistakes estate planning specialists see most often among HNW families who thought they had it handled.

Mistake 1: Skipping the portability election after a spouse dies

When a spouse dies, any unused portion of their federal estate tax exemption can transfer to the surviving spouse — this is called the Deceased Spouse's Unused Exclusion (DSUE) under IRC §2010(c). A spouse who died in 2025 with a $4M estate had $9.99M in unused exemption ($13.99M − $4M). The surviving spouse can add that to their own $15M exemption in 2026 for a combined shelter of nearly $25M.

But portability is not automatic. The executor must file Form 706 within 9 months of death (extendable to 15 months with IRS Form 4768) — even if the estate owes zero tax.¹ Skip it, and the DSUE is gone forever.

Important nuance: the DSUE amount is locked in at the exemption level in effect when the first spouse died. It is not re-indexed upward if the exemption later increases. And only the most recently deceased spouse's DSUE counts — serial marriages require careful attention to which DSUE applies.

Mistake 2: Confusing a revocable living trust with estate tax planning

A revocable living trust (RLT) is a probate-avoidance tool — it keeps assets out of probate court at death and lets a successor trustee act without court intervention during incapacity. Those are real benefits. But an RLT does nothing to reduce federal or state estate tax.

Because you retain full control over a revocable trust, the IRS treats every asset inside it as part of your taxable estate — exactly as if you held those assets in your personal name. At $10M+ net worth, estate tax reduction requires irrevocable structures: a SLAT to use the lifetime exemption while retaining indirect access through your spouse, a zeroed-out GRAT to transfer expected appreciation at near-zero gift-tax cost, an IDGT installment sale for closely-held business interests, or a dynasty trust to shelter assets from estate tax at every generation's death.

Many families have the revocable trust conversation with an attorney, leave feeling like they've "done estate planning," and stop there. The advisor's job is to make clear that the revocable trust is the foundation — not the plan. See: Trust Strategies Compared: IDGT, GRAT, SLAT, Dynasty, QPRT.

Mistake 3: Keeping appreciated assets in an irrevocable trust — and forfeiting the step-up in basis

When you transfer an appreciated asset to an irrevocable trust, you're removing it from your taxable estate — which saves estate tax. But you also cause it to lose the step-up in cost basis that would otherwise occur at your death under IRC §1014. Your heirs inherit the asset at your original cost basis, not the date-of-death fair market value, and they owe capital gains tax on all the appreciation when they eventually sell.

For highly appreciated assets — a business worth $5M with a $200K basis, or real estate held for 30 years — the capital gains exposure can be enormous. At 23.8% combined federal rate (20% LTCG + 3.8% NIIT), the tax on $4.8M of gain is over $1.1M. The estate tax saved by removing a $5M asset from a $20M estate is $2M. In this example the irrevocable trust wins — but the math isn't always that clear.

The right framework: model both paths before transferring any asset. See: Step-Up Basis Impact Calculator. For assets you want to transfer but also want to benefit charity, a charitable remainder trust (CRT) offers a middle path — the trust sells tax-free, pays you income, and removes the asset from your estate while directing the remainder to charity.

Mistake 4: The unfunded trust

This is the most common mistake in estate planning — and the most frustrating, because the fix is simple. An estate plan is drafted, documents are signed, attorneys are paid, and nothing actually changes. The house is still titled in personal names. The brokerage account still says "John Smith," not "The Smith Family Trust." The LLC interest was never assigned to the trust.

At death, every asset not titled in the trust or with a proper beneficiary designation goes through probate — defeating the entire purpose of the revocable trust. Worse, an unfunded irrevocable trust may not have met the legal requirements for a completed gift, undermining the estate tax strategy entirely.

The funding checklist every HNW family should complete:

• Real estate: re-title deeds into the trust name (or into an LLC held by the trust). Requires new deeds in each county and state of ownership.
• Taxable brokerage accounts: retitle or set up transfer-on-death (TOD) to the trust.
• Business interests (LLCs, limited partnerships): assignment of membership or partnership interests — requires operating agreement review and sometimes consent of co-owners.
• Life insurance: name the irrevocable life insurance trust (ILIT) as owner and beneficiary — not the revocable trust, which would bring proceeds back into the estate.
• Retirement accounts: do NOT retitle into the trust (causes taxable distribution). Instead, name correct beneficiaries on each account directly.

Mistake 5: Beneficiary designations that override the will — and haven't been updated

Retirement accounts (IRA, 401(k), 403(b), pension), life insurance policies, annuities, and transfer-on-death (TOD) accounts pass entirely outside the will and trust. The beneficiary designation on file with the custodian controls 100% — regardless of what the will or trust says.

This creates problems in predictable life situations:

• Divorce: an ex-spouse named as IRA beneficiary 15 years ago is still the legal beneficiary today. The divorce decree doesn't automatically update custodian records.
• Death of a named beneficiary: if a child named as primary beneficiary predeceased you and no contingent beneficiary is named, the account may pass through probate — or to your estate — instead of to your grandchildren as intended.
• Newly born grandchildren: beneficiary designations from before they existed won't include them.
• Unintended large IRA inheritance by a minor: requires court appointment of a guardian of the property to manage distributions.

Action: request a beneficiary designation audit from your advisor. Review every custodian-held account annually. At minimum: IRA, 401(k), 403(b), life insurance (all policies), HSA, annuity contracts, TOD brokerage accounts.

Mistake 6: Using exemption on the wrong assets (low-growth instead of high-growth)

The lifetime gift and estate tax exemption ($15M per person in 2026) is finite. Once used, it's gone — and so is the chance to shelter that amount from the 40% estate tax. The highest-leverage use of exemption is to move assets with high expected future appreciation out of the estate today, so all that future growth occurs outside the taxable estate.

A common mistake: gifting cash or Treasury bonds to a SLAT. Those assets appreciate slowly. You've used $1M of exemption to remove $1.1M from your estate over five years — not terrible, but suboptimal.

Better uses of exemption on high-appreciation assets:

• Pre-IPO equity or closely-held business shares: fund a zeroed-out GRAT — if the business doubles in value over the GRAT term, you've transferred that $X million in appreciation with near-zero gift-tax cost, using no lifetime exemption at all.
• Rental real estate with growth potential: fund a SLAT while the property is early in its appreciation cycle; all future rents and appreciation stay out of your estate.
• IDGT installment sale: sell business interest to an intentionally defective grantor trust at fair market value; trust pays you interest-only notes using IRC §7872 rates (~4.6% in April 2026), and all appreciation above that rate passes to heirs free of gift and estate tax.

Mistake 7: Building a SLAT — then violating the reciprocal trust doctrine

SLATs (Spousal Lifetime Access Trusts) are popular because they let married couples remove assets from the estate while retaining indirect access through the beneficiary spouse. But a predictable trap: both spouses set up essentially identical SLATs for each other on the same schedule.

The IRS reciprocal trust doctrine (established in United States v. Grace, 395 U.S. 316 (1969)) says that if two trusts are sufficiently interrelated, courts will unwind them — treating each spouse as the effective owner of the trust the other funded. The entire strategy collapses.

How to differentiate two SLATs:

• Stagger funding dates by at least 6–12 months
• Use different trustees (one independent corporate trustee, one family-friend trustee)
• Use different distribution standards (HEMS on one, pure discretionary on the other)
• Use different asset classes to fund each trust
• Give the trusts different duration/term structures

Two SLATs can work — but they need deliberate differentiation in both drafting and execution. See: SLAT vs. GRAT comparison calculator.

Mistake 8: No systematic annual gifting program

The annual gift tax exclusion is $19,000 per donor per recipient in 2026.³ That sounds modest — but consistency over time turns it into a powerful estate-reduction machine.

A family with two parents and four married children has 8 potential donees (4 children + 4 spouses-in-law). At $19,000 per donor per donee:

• Mother's gifts: 8 × $19,000 = $152,000/year
• Father's gifts: 8 × $19,000 = $152,000/year
• Combined: $304,000/year removed from the taxable estate — using no lifetime exemption
• Over 20 years: $6.08M transferred. Assuming the estate would have grown those dollars at 6%/year, the compounded estate reduction is closer to $11M.

Add 529 superfunding ($95,000 per donor per grandchild, front-loading 5 years of annual exclusion) for each grandchild, and the numbers climb further.

The mistake isn't misunderstanding the annual exclusion — it's failing to implement a systematic program. Many families make occasional gifts informally but never have a gifting calendar, a target recipient list, or a coordination process with their advisor and accountant to make sure gifts are documented and not double-counted. See: Annual Gift Tax Exclusion Calculator.

Mistake 9: Ignoring state estate tax — especially the New York cliff rule

The federal estate tax exemption is $15M in 2026. But 17 jurisdictions (12 states + DC + some inheritance-tax states) impose their own estate or inheritance tax with much lower exemptions — some as low as $1M.

Two especially painful traps:

New York's cliff rule. New York's estate tax exemption is $7.16M in 2026. If your estate exceeds that threshold by even $1, the entire estate is subject to New York estate tax from dollar one — not just the amount above the threshold. An estate of $7.17M owes New York estate tax on the full $7.17M (roughly $880K). An estate of $7.15M owes zero. This cliff effect at the margin is unique and creates significant planning urgency for NY-domiciled families in the $7M–$9M range.

Non-portability. Most states with estate taxes do not offer portability between spouses. The deceased spouse's unused exemption is simply lost — there's no state-level equivalent of DSUE. Married couples in those states often benefit from a credit shelter trust (bypass trust) to fully use both spouses' exemptions at first death, rather than relying on portability (which isn't available at the state level).

See the full guide: State Estate Tax 2026 — All 17 Jurisdictions.

Mistake 10: No coordination between financial advisor, attorney, and CPA

Estate planning is inherently multi-discipline. The attorney drafts the trust documents. The CPA handles the gift tax returns (Form 709), the estate tax return (Form 706), and income tax implications. The financial advisor manages the portfolio strategy, models the trust transfer scenarios, and executes the funding. Each professional is competent in their lane — but the lanes are adjacent and highly interdependent.

What happens without coordination:

• Trust document drafted to hold $X in assets — but financial advisor doesn't know the terms and funds the trust with assets that don't fit the distribution standard.
• GRAT structured around an asset class the CPA reports differently than the attorney assumed — creating a taxable event neither anticipated.
• Irrevocable trust funded with retirement assets by mistake — triggering immediate income tax on the full distribution.
• Annual gifting program undocumented — CPA files Form 709 late or misses prior-year gifts, creating audit exposure.
• Life insurance policy owned by the wrong entity — premiums paid from the estate rather than outside it, keeping proceeds in the taxable estate at death.

The advisor's role in this team is often to quarterback: translate the attorney's trust language into portfolio reality, model scenarios before documents are signed, and make sure the financial strategy and the legal strategy are consistent. If your financial advisor and estate attorney have never had a direct conversation about your plan, that's a red flag — not a normal state of affairs.

Sources

1. IRS — Frequently Asked Questions on Estate Taxes (Form 706 portability election deadline: 9 months from date of death, extendable 6 months via Form 4768; IRC §2010(c))
2. IRS Revenue Procedure 2022-32 (simplified late-portability election up to 5 years from date of death for estates below the filing threshold)
3. IRS — Tax Year 2026 Inflation Adjustments (including OBBBA) (annual gift exclusion $19,000/donee; estate exemption $15,000,000; non-citizen spouse annual exclusion $194,000)
4. IRS — What's New in Estate and Gift Tax (DSUE portability under IRC §2010(c); OBBBA permanent $15M exemption)

Values verified as of April 2026: annual gift exclusion $19,000 (IRS 2026 inflation adjustments); federal estate exemption $15,000,000 (OBBBA, July 2025); Rev. Proc. 2022-32 late-portability 5-year window (IRS, July 2022); LTCG 20% + NIIT 3.8% (IRC §1411, 2026 brackets).