Rent vs Buy Total Cost of Ownership Tutor — Mortgage, Tax, Insurance, Maintenance + NPV Break-Even and IRR

The model runs month by month: the fixed-rate payment splits into interest and principal; property tax and maintenance grow with the appreciating value; the tax shield is marginalRate × max(0, mortgage interest + min(property tax, SALT cap) − standard deduction) — the incremental itemize-vs-standard comparison (2026 SALT cap $40,400 with a high-income phaseout; mortgage interest is only deductible on the first $750,000 of principal). Renter wealth compounds (down payment + closing costs) plus the monthly rent-buy difference at the investment return; break-even is the first year net sale proceeds (after selling costs) overtake the renter's account; differential NPV discounts the extra money buying requires, so NPV > 0 means buying beats the alternative investment; the IRR is solved with Newton-Raphson and a bisection fallback. Price-to-rent bands: below 15 favours buying, 15–20 balanced, above 20 favours renting, and the 5% rule gives the parity rent for unrecoverable costs. Capital-gains exclusion (IRC §121) is not modelled. Educational tool — not tax or investment advice.