The optimal mortgage amount hinges on a delicate balance of your financial assets, wealth, human capital, and risk tolerance. When determining how much of your investment portfolio to allocate to your home, consider your long-term financial goals to balance risk and return, and assess your overall risk tolerance. Real estate investments, such as your home, offer stability and potential appreciation but are also illiquid and typically involve leverage. Households often accumulate financial wealth for a down payment, leading to a significant shift in asset allocation in the year of purchase.
Real estate is inherently less liquid than equities, meaning it can take longer to sell and convert into cash. This lack of liquidity is compounded by market inefficiencies and idiosyncratic risk, which refers to the unique risks associated with individual properties or local real estate markets. These risks include factors like location desirability, property condition, and local economic conditions, all of which can affect a property’s value62 and salability. Most households cannot diversify away from risks specific to their home’s capital gains, unlike institutional investors who can partially diversify such risks in private equity and commercial real estate. This idiosyncratic risk not only reduces the Sharpe ratio of housing investments but also makes these ratios dependent on the holding period. Longer holding periods generally offer a more favorable risk-return trade-off due to the term structure of idiosyncratic risk.
In different geographic areas, the impact of idiosyncratic risk varies. Wealthier areas typically exhibit low price index volatility but high idiosyncratic risk. Without considering this idiosyncratic component, one might mistakenly believe that high-income zip codes provide exceptionally high Sharpe ratios for local homeowners. However, when idiosyncratic risk is accounted for, the apparent differences in Sharpe ratios between affluent and less affluent areas diminish.
The idiosyncratic component of capital gains, which accounts for factors such as capital investment, local market fluctuations, and house characteristics, is a primary driver of capital gains variance across most of California’s zip codes for holding periods up to 10 years. Even after 15 years, it represents between 25% and 60% of the variance, depending on the location. The influence of this component is expected to decrease over time with advancements in property technology platforms like Zillow, which improve market liquidity and facilitate transactions.
The mortgage a liability against your asset (the home). Yet, the interest rate on a mortgage is typically lower than other types of borrowing, making it a relatively cost-effective way to leverage. A home mortgage is often likened to a “negative bond” due to its fixed payment structure, similar to the regular payouts of a bond. However, the fundamental difference is that while a bond generates interest income for the holder, a mortgage requires the borrower to pay interest to the lender. When you make additional payments towards the mortgage principal, it’s akin to investing in bonds because it reduces your debt (similar to increasing your assets) and decreases the interest expense over time. This action effectively increases your equity, and thus, should be considered part of your equity allocation in financial planning. If you have or plan mortgage — reduce your bond allocation accordingly, and account for downpayment, home equity as a real estate. This way mortgages may diminish the benefits of equity market participation by offering an alternative risk-free rate, while at the same time low correlation between stock and housing returns suggests higher risky asset share in financial portfolios for diversification benefits post-house purchase if mortgage interest is lower than stock market returns.
The equity in your home is considered an asset that contributes to your net worth. It increases as you pay down the mortgage or as the property value appreciates. However, it is important to account for selling costs when calculating your net worth. Furthermore, be aware that equity can decrease and potentially go “underwater” during market downturns. This equity can be borrowed against, typically at favorable rates, providing a source of funds when needed. However, there is a trade-off between accelerating mortgage repayment and investing in low-risk fixed income assets, like government bonds from high credit-rating countries. The decision here often hinges on the comparative expected return of the investments versus the interest saved by paying down the mortgage.
Purchasing a home involves opportunity costs, as the down payment and additional expenses — such as larger mortgage payments, homeowner’s insurance, and maintenance costs — could be invested elsewhere, typically in stocks or bonds. These expenses can significantly reduce your ability to save and invest, potentially impacting the growth of your overall investments. The decision should align with your broader financial objectives, such as retirement planning, wealth accumulation, or debt reduction.
The impact of mortgage interest rates is central to this analysis. Historically, when mortgage rates are below 4%, the cost of borrowing is relatively low compared to the average return from the stock market, which has historically returned about 7% annually. This disparity suggests that, in scenarios where mortgage rates are below 4%, the financial burden of a mortgage is offset by the lower cost of borrowing and the potential for higher returns from home appreciation.
If you choose to rent and invest all potential savings (the money that would have gone toward a downpayment and other homeownership costs), you might achieve higher liquidity and possibly higher returns, depending on market conditions. However, this scenario lacks the potential benefits of home equity and the forced savings mechanism inherent in making regular mortgage payments.
The Case-Shiller House Price Index tracks U.S. residential prices across 20 metropolitan regions using a repeat-sales method — comparing successive sales of the same properties — which makes it a clean benchmark for measuring housing against stocks and bonds (see Figure 11.4 and Figure 11.5).
Using realized Case-Shiller returns rather than appraisal-based ones, Wu and Pandey (2012)63 computed the optimal portfolio weight of residential real estate across 20 metropolitan areas over 1987–2010. Their result is the one most homeowners would rather not hear: for residents of most cities, the optimal portfolio is weighted heavily toward equities, then bonds, with residential real estate carrying only a small weight. The exceptions — Washington DC, Los Angeles, Portland, San Diego, and San Francisco, where housing optimally runs 60–90% of the weights — are precisely the high-appreciation coastal metros, and even there the finding leans on a historical run that need not repeat.
Here is the uncomfortable gap. Theory says housing should be a minor slice of an optimal portfolio; in practice, the typical household holds the majority of its net worth in a single home. That gap is the central portfolio problem with a primary residence, and it has three compounding causes:
One property, one street, one city — a single, undiversified asset, often the largest you will ever own. Its idiosyncratic risk, discussed above, cannot be diversified away the way a stock position can.
You cannot sell 5% of the house when it drifts above its target weight. The position cannot be rebalanced, so every other holding must do the rebalancing work.
A home’s value and the owner’s paycheck often ride on the same local economy, so a regional downturn can strike job and home equity together — the concentration risk examined in the buy-versus-rent discussion (section “Rent vs Buy decision”).
None of this argues against owning a home. It argues for placing the home honestly inside the allocation rather than leaving it off the books:
Used this way, residential real estate also offers a modest hedge against inflation across most historical periods — a genuine but secondary benefit, not a reason to overweight it.
