Documenting Assumptions in a Financial Plan

Every financial plan rests on assumptions about the future. Investment returns, inflation rates, life expectancy, and tax rates all involve uncertainty. Documenting these assumptions explicitly serves two purposes: it makes the plan's foundation transparent, and it creates a framework for testing how sensitive your outcomes are to changes in those assumptions.

Core Assumption Categories

Inflation Assumptions

Inflation erodes purchasing power over time. A dollar today buys less in the future.

Historical context:

• U.S. average inflation 1926-2023: 2.9% annually
• U.S. average inflation 2000-2023: 2.5% annually
• Federal Reserve target: 2.0% annually

Reasonable assumption ranges:

• Conservative (higher costs): 3.0-3.5%
• Moderate (historical average): 2.5-3.0%
• Optimistic (lower costs): 2.0-2.5%

Category-specific inflation: Healthcare costs have historically risen faster than general inflation (5-7% annually). Education costs have risen 4-6% annually. Housing costs vary significantly by location.

Documentation example: "This plan assumes 2.5% general inflation, 5.5% healthcare cost inflation, and 4.0% education cost inflation."

Investment Return Assumptions

Return assumptions drive projections for wealth accumulation and retirement income sustainability.

Historical nominal returns (1926-2023):

• U.S. large-cap stocks: 10.3% annually
• U.S. small-cap stocks: 11.8% annually
• U.S. long-term government bonds: 5.5% annually
• U.S. Treasury bills: 3.3% annually

Real returns (after inflation):

• U.S. large-cap stocks: 7.2% annually
• U.S. bonds: 2.4% annually

Forward-looking considerations: Current market valuations, interest rate environment, and economic conditions suggest many analysts project lower returns for the next decade than historical averages.

Reasonable assumption ranges for diversified portfolios:

Documentation example: "This plan assumes 6.0% real (inflation-adjusted) returns on a 70/30 stock/bond portfolio during the accumulation phase, reducing to 5.0% real returns after age 60 as allocation becomes more conservative."

Life Expectancy Assumptions

Underestimating longevity creates the risk of outliving your money.

Current life expectancy data (U.S.):

• At birth: 76.4 years (2023)
• At age 65: Additional 18.5 years (to age 83.5)
• At age 65 for couples: 50% chance one survives to age 92

Planning recommendations: Individual planning: Age 90-92 Couple planning: Age 95 (for the longer-lived spouse) Conservative planning: Age 95-100

Documentation example: "This plan models income needs through age 95 for the surviving spouse, with healthcare cost estimates extending through that age."

Tax Assumptions

Tax rates affect both accumulation (how much you keep) and withdrawal (how much you net from retirement accounts).

Current federal tax context (2024):

• Top marginal rate: 37%
• Capital gains (long-term): 0%, 15%, or 20% depending on income
• Qualified dividends: Same as long-term capital gains
• Standard deduction: $14,600 (single), $29,200 (married filing jointly)

Reasonable assumption approaches:

• Assume current rates continue (legislative risk)
• Assume rates increase by 2-5 percentage points
• Assume your marginal rate in retirement based on expected income

Documentation example: "This plan assumes federal tax rates remain at current levels through 2030, then increase by 3 percentage points across all brackets. State tax rate assumed at current 5.75% throughout."

Healthcare Cost Assumptions

Healthcare represents a significant and growing expense, particularly in retirement.

Benchmark data:

• Fidelity estimate for couple retiring at 65 (2023): $315,000 lifetime healthcare costs
• Medicare Part B premium (2024): $174.70/month standard
• Medigap policies: $150-$400/month depending on coverage

Documentation example: "Healthcare costs modeled at $8,000/year pre-65, increasing to $15,000/year from ages 65-75, $20,000/year from 75-85, and $30,000/year from 85-95, all amounts in today's dollars inflated at 5.5% annually."

Social Security Assumptions

Social Security provides a foundation of retirement income but faces funding challenges.

Current status:

• Trust fund projected depletion: 2034 (per 2023 Trustees Report)
• Projected benefit if no changes: 77% of scheduled benefits after depletion

Assumption approaches:

• Full scheduled benefits (optimistic)
• 75-80% of scheduled benefits (moderate)
• Delay claiming assumptions to age 67 or 70
• Exclude Social Security entirely (most conservative)

Documentation example: "Social Security benefits modeled at 80% of currently projected amounts, with claiming at age 67 for both spouses."

Worked Example: Sensitivity Analysis

Sensitivity analysis shows how outcomes change when assumptions vary. This reveals which assumptions matter most to your plan's success. To keep the comparison consistent, this entire example is in real (today's dollar) terms: returns are real (after inflation), contributions hold their purchasing power over time, and so does projected spending. That avoids the common error of mixing nominal projections with real spending targets.

Base Case Scenario:

• Current age: 40
• Retirement age: 65 (25-year accumulation horizon)
• Current retirement savings: $400,000
• Annual savings: $30,000 (real, growing with inflation)
• Investment return: 6.5% real
• Retirement spending: $80,000/year (today's dollars)
• Life expectancy: Age 92
• Social Security: $30,000/year at age 67 (today's dollars)

Base case projection (compounding $400K start + $30K/yr at 6.5% real over 25 years):

• Portfolio at age 65: ~$3,700,000 (real)
• Sustainable real withdrawal at 4%: $148,000/year
• Plus Social Security: $30,000/year
• Total real income: $178,000/year
• Spending need: $80,000/year
• Result: Substantial surplus — the plan is over-funded under the base assumptions, suggesting room to retire earlier, save less, or plan for higher spending.

Testing Return Assumption Sensitivity

Analysis: Each 1 percentage-point decrease in real returns reduces the projected portfolio by roughly $500–800K and sustainable income by $20–30K/year. The plan is robust to a wide return range under these contributions.

Testing Inflation Sensitivity

Working in real terms isolates inflation's effect to two places: how fast healthcare and other category-specific costs outpace general inflation, and whether your contributions actually keep up with inflation in nominal dollars. The table below shows what $80,000 of real spending looks like in nominal dollars at age 65 — useful only to translate the plan back into the dollar amounts you'll see on statements:

Analysis: The nominal numbers grow quickly, but if returns and contributions are also expressed in real terms (as above), inflation's first-order effect on the plan is already captured. The places to stress-test inflation separately are (a) categories that outpace it — healthcare and education — and (b) Social Security, which is COLA-adjusted but where future legislative changes could erode purchasing power.

Testing Longevity Sensitivity

Holding the $80K real spending need fixed and Social Security at $30K real, the portfolio required at age 65 to fund that spending over different horizons (using a 0% real return assumption as a conservative amortization, since safe withdrawal rates fall as horizons lengthen):

Analysis: Each 5-year extension of the planning horizon adds roughly $250,000 to the required portfolio. The base-case projection of ~$3.7M comfortably covers all of these — the plan has meaningful longevity headroom.

Combined Stress Test

Worst reasonable case (all assumptions adverse simultaneously):

• Returns: 4.5% real → portfolio ~$2,540,000 (real)
• Plan through age 95 (30-year retirement horizon)
• Social Security: 75% of projected = $22,500/year (real)

Result:

• Portfolio at 65: ~$2,540,000 (real)
• Sustainable real withdrawal over 30 years at ~3.5% (a longevity-adjusted SWR): ~$89,000/year
• Plus Social Security at 75%: $22,500/year
• Total real income: ~$111,500/year
• Spending need: $80,000/year
• Result: Surplus of ~$31,000/year — the plan survives even in the combined-stress scenario.

The lesson worth taking from a sensitivity analysis like this: the assumptions that look the scariest in isolation (a future Social Security cut, longevity to age 100, a 200 bps real return shortfall) are often individually survivable. The combinations matter, and the response is to identify which assumption is doing the most work in your plan and stress that one hardest. In this base case, returns dominate — a sustained run of 3% real returns instead of 6.5% real would be the single change that most threatens the plan.

Response options if the stress test fails for your numbers:

1. Increase savings rate
2. Delay retirement (the highest-leverage lever — adds compounding, reduces withdrawal years)
3. Reduce planned spending
4. Plan for part-time work in early retirement
5. Re-examine which assumption is driving the failure and stress-test that one specifically before changing the plan

Documenting Assumptions Format

Create a dedicated section in your financial plan with this structure:

Assumption Documentation Template:

Review Cadence for Assumptions

Annual Review (During Comprehensive Plan Review)

Check each assumption against:

• New data (actual inflation, market returns)
• Updated projections from credible sources
• Changes in personal circumstances
• Legislative or regulatory changes

Update triggers:

• Actual results differ from assumptions by more than 1% for two consecutive years
• Major policy changes (tax law, Social Security reform)
• Significant life changes (health diagnosis, inheritance, divorce)

Sources for Assumption Updates

Inflation: Bureau of Labor Statistics CPI data, Federal Reserve projections Investment returns: Vanguard Capital Markets Model, Research Affiliates, Morningstar Life expectancy: Social Security Administration tables, Society of Actuaries Tax rates: IRS announcements, Congressional Budget Office projections Healthcare costs: Kaiser Family Foundation, CMS National Health Expenditure data

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Assumptions Documentation Checklist

• Documented inflation assumption with source
• Specified separate healthcare inflation rate
• Stated investment return assumption for accumulation phase
• Stated investment return assumption for retirement phase
• Documented whether returns are nominal or real (inflation-adjusted)
• Specified life expectancy assumption for planning
• Documented Social Security benefit assumption and claiming age
• Stated federal tax rate assumptions
• Stated state tax rate assumptions
• Listed any income growth assumptions
• Documented healthcare cost projections
• Created date stamp for when assumptions were set
• Ran sensitivity analysis on return assumptions
• Ran sensitivity analysis on inflation assumptions
• Ran sensitivity analysis on longevity assumptions
• Tested worst reasonable case scenario
• Identified response options if stress test fails
• Scheduled annual assumption review date
• Listed sources for future assumption updates