Ownership cost
Principal, interest, the reassessed tax, the indexed premium, HOA exposure and a reserve line — five rows, not one number.
Monthly outflow, year one
Computed
The purchase
A share of price, so 10 means ten percent.
Default is the 30-year average for the week of 2026-10-24.
The carrying cost
Against market value. Your tax bill wins over the seeded range.
Your quote. The national average is a placeholder, not a forecast.
From the reserve study. Below 70%, the engine spreads a modelled assessment over three years.
Sets the reserve row. The sinking-fund engine lets you age each system separately.
Year one, line by line
7.32% on $375,750
Maryland — Montgomery County at 1.02% effective
Your quote, not a national average
Five-year hazard model, not 1% of value
Where the payment goes
| Year | Monthly | Tax | Insurance |
|---|---|---|---|
| Year 1 | $3,746 | $357 | $255 |
| Year 5 | $4,017 | $417 | $347 |
| Year 10 | $4,456 | $508 | $509 |
Escrow inflates faster than the mortgage: 4% on tax, 8% on insurance, 5% on the reserve.
Triennial reassessment in groups; the county rate is set annually on top of the state rate.
Tax-Property Article §8-104; SDAT notices
The reserve row is a hazard model, not a rule of thumb: a 1998 house with original systems carries a different monthly cost than a 2019 house with none.
In an acquisition-value state the assessment resets on sale. The listing prints the seller’s tax, which may reflect a basis set decades ago; the buyer pays on the price. The engine computes the reassessed figure and shows the delta as its own row.
Carriers do not publish a national average that applies to a specific house. The seeded figure is a starting point, and the page says so. Replace it with the quote you were given, because that is the only number the escrow will use.
The one-percent rule assumes every system in the house is the same age. It is not. The reserve row comes from the same wear-out model the sinking-fund engine uses, so the age of the roof and the furnace change the monthly cost.
outflow = (P + I) + T_reassessed + I_indexed + HOA + M_predictive
T_reassessed = price × effective rate / 12
HOA = dues + (reserve funding < 70% ? assessment / 36 : 0)
M_predictive = Σ(replacement × hazard) / 12
Eight jurisdictions are modelled: Texas, California, Cook and non-Cook Illinois, Ohio, Georgia, New York municipalities and Montgomery County, Maryland. Elsewhere the engine refuses rather than substituting a national average.