{ "openapi": "3.1.1", "info": { "title": "Ledgerly Rent vs Buy Calculator API", "version": "1.0.0", "summary": "Rent or buy? Break-even year, net worth over time, scenarios and a sensitivity table.", "description": "Every endpoint performs the same calculation as the human page and returns inputs, result, units, assumptions, sources, warnings and computedAt. MCP endpoint: /mcp." }, "servers": [ { "url": "https://ledgerlyrentvsbuy.usefulweb.app" } ], "paths": { "/api/v1/calculate": { "post": { "operationId": "calculate", "summary": "Rent or buy? Break-even year, net worth over time, scenarios and a sensitivity table.", "description": "Simulates buying and renting month by month: mortgage schedule, buying and selling costs, property tax, maintenance, insurance, rent increases and the return the renter earns by investing the down payment and every monthly difference. Shows the break-even year, net worth on both sides, home-price scenarios and a sensitivity table. Part of Ledgerly’s Reality Check.\n\nMethodology:\n- The mortgage is a monthly annuity built with the same schedule engine as the Loan Amortization Calculator (integer cents, interest rounded per month).\n- Buying needs the down payment plus buying costs in cash; the renter invests exactly that amount instead. Each month the side with the lower costs invests the difference, so both sides spend the same money.\n- Owner costs: mortgage payment, property tax and maintenance (percent of the home value at the start of each year), insurance and other fees (growing with the cost increase). Renter costs: rent (growing with the rent increase) and renter’s insurance.\n- Net worth in year N: buyer = home value − selling costs − tax on the sale gain − remaining mortgage + invested surplus after tax; renter = invested savings after tax on the gains. The break-even year is the first year buying is ahead; “not stable” means it falls behind again later.\n- Scenarios move the home price growth by ± the scenario step; the sensitivity table varies price growth (rows, ±2 steps) and the investment return (columns, ±2 percentage points).\n\nA model, not advice: house prices, rents and returns are uncertain, and local taxes and subsidies differ. Change the inputs to match your offer and check the sensitivity table before deciding.", "tags": [ "apps" ], "x-mcp-tool": "ledgerly_rent_vs_buy", "x-owner-app": "ledgerly", "requestBody": { "required": true, "content": { "application/json": { "schema": { "$ref": "#/components/schemas/LedgerlyRentVsBuyInput" }, "examples": { "example1": { "summary": "€380k flat vs €1,450 rent, 20 % down, 30 years", "value": { "currency": "EUR", "homePrice": 380000, "downPaymentPercent": 20, "buyingCostsPercent": 10, "mortgageRatePercent": 3.6, "mortgageYears": 30, "rentPerMonth": 1450, "horizonYears": 30 } }, "example2": { "summary": "US house: $450k, 10 % down, property tax 1.1 %", "value": { "currency": "USD", "homePrice": 450000, "downPaymentPercent": 10, "buyingCostsPercent": 3, "mortgageRatePercent": 6.5, "mortgageYears": 30, "propertyTaxPercent": 1.1, "maintenancePercent": 1, "ownerInsurancePerYear": 1800, "appreciationPercent": 3, "sellingCostsPercent": 6, "rentPerMonth": 2400, "rentIncreasePercent": 3, "investmentReturnPercent": 6, "investmentTaxPercent": 15, "horizonYears": 15 } }, "example3": { "summary": "Short stay: selling again after 5 years", "value": { "homePrice": 300000, "downPaymentPercent": 25, "buyingCostsPercent": 9, "mortgageRatePercent": 3.8, "rentPerMonth": 1100, "horizonYears": 5 } } } } } }, "responses": { "200": { "description": "Calculation result envelope.", "content": { "application/json": { "schema": { "$ref": "#/components/schemas/LedgerlyRentVsBuyResult" } } } }, "400": { "description": "Malformed JSON body.", "content": { "application/problem+json": { "schema": { "$ref": "#/components/schemas/Problem" } } } }, "422": { "description": "Invalid input (see errors[]).", "content": { "application/problem+json": { "schema": { "$ref": "#/components/schemas/Problem" } } } }, "429": { "description": "Rate limit exceeded (see Retry-After).", "content": { "application/problem+json": { "schema": { "$ref": "#/components/schemas/Problem" } } } } } } }, "/api/v1/tool": { "get": { "operationId": "getToolDefinition", "summary": "Inputs, output schema and examples for this tool", "responses": { "200": { "description": "Tool definition.", "content": { "application/json": { "schema": { "type": "object" } } } } } } } }, "components": { "schemas": { "Problem": { "type": "object", "description": "RFC 9457 problem details. 422 responses include per-field `errors`.", "required": [ "type", "title", "status", "detail" ], "properties": { "type": { "type": "string" }, "title": { "type": "string" }, "status": { "type": "integer" }, "detail": { "type": "string" }, "code": { "type": "string" }, "errors": { "type": "array", "items": { "type": "object", "required": [ "field", "message" ], "properties": { "field": { "type": "string" }, "message": { "type": "string" } } } } } }, "LedgerlyRentVsBuyInput": { "type": "object", "additionalProperties": false, "properties": { "currency": { "title": "Currency", "description": "ISO code used for the labels, e.g. EUR, USD, CHF.", "type": "string", "maxLength": 3, "default": "EUR" }, "homePrice": { "title": "Purchase price of the home", "type": "number", "minimum": 1000, "maximum": 100000000, "default": 380000 }, "downPaymentPercent": { "title": "Down payment (% of the price)", "type": "number", "minimum": 0, "maximum": 100, "default": 20 }, "buyingCostsPercent": { "title": "Buying costs (% of the price)", "description": "Transfer tax, notary, land registry and agent fee, paid in cash on top of the down payment.", "type": "number", "minimum": 0, "maximum": 30, "default": 10 }, "mortgageRatePercent": { "title": "Mortgage interest rate (% per year)", "type": "number", "minimum": 0, "maximum": 25, "default": 3.6 }, "mortgageYears": { "title": "Mortgage term (years)", "type": "integer", "minimum": 1, "maximum": 40, "default": 30 }, "propertyTaxPercent": { "title": "Property tax (% of the home value per year)", "type": "number", "minimum": 0, "maximum": 5, "default": 0.3 }, "maintenancePercent": { "title": "Maintenance (% of the home value per year)", "type": "number", "minimum": 0, "maximum": 10, "default": 1 }, "ownerInsurancePerYear": { "title": "Home insurance per year", "type": "number", "minimum": 0, "maximum": 1000000, "default": 600 }, "ownerFeesPerMonth": { "title": "Other owner costs per month", "description": "For example service charges or a home-owners association fee that is not maintenance.", "type": "number", "minimum": 0, "maximum": 100000, "default": 0 }, "appreciationPercent": { "title": "Home price growth (% per year)", "type": "number", "minimum": -10, "maximum": 15, "default": 2 }, "sellingCostsPercent": { "title": "Selling costs at the end (% of the value)", "type": "number", "minimum": 0, "maximum": 20, "default": 6 }, "saleGainTaxPercent": { "title": "Tax on the gain when selling (%)", "description": "Often 0 for an owner-occupied home; enter your local rate if a gain is taxed.", "type": "number", "minimum": 0, "maximum": 70, "default": 0 }, "rentPerMonth": { "title": "Rent per month (comparable home)", "type": "number", "minimum": 0, "maximum": 1000000, "default": 1450 }, "rentIncreasePercent": { "title": "Rent increase (% per year)", "type": "number", "minimum": -10, "maximum": 20, "default": 2.5 }, "renterInsurancePerYear": { "title": "Renter’s insurance per year", "type": "number", "minimum": 0, "maximum": 100000, "default": 150 }, "costIncreasePercent": { "title": "Increase of insurance and fees (% per year)", "type": "number", "minimum": -10, "maximum": 20, "default": 2 }, "investmentReturnPercent": { "title": "Return on invested savings (% per year)", "description": "The opportunity cost: what the down payment and every monthly difference would earn if invested instead.", "type": "number", "minimum": -10, "maximum": 20, "default": 5 }, "investmentTaxPercent": { "title": "Tax on investment gains (%)", "type": "number", "minimum": 0, "maximum": 70, "default": 25 }, "horizonYears": { "title": "Years to compare", "type": "integer", "minimum": 1, "maximum": 40, "default": 30 }, "scenarioStepPercent": { "title": "Scenario step for home price growth (percentage points)", "type": "number", "minimum": 0.25, "maximum": 5, "default": 1 } }, "required": [] }, "LedgerlyRentVsBuyResult": { "type": "object", "additionalProperties": false, "required": [ "schemaVersion", "tool", "computedAt", "inputs", "summary", "result", "units", "assumptions", "sources", "warnings", "presentation", "links" ], "properties": { "schemaVersion": { "type": "string", "const": "1.0" }, "tool": { "type": "object", "additionalProperties": false, "required": [ "id", "name", "version" ], "properties": { "id": { "type": "string" }, "name": { "type": "string" }, "version": { "type": "string" } } }, "computedAt": { "type": "string", "format": "date-time" }, "inputs": { "type": "object", "description": "Normalised inputs actually used, including applied defaults." }, "summary": { "type": "string" }, "result": { "type": "object", "additionalProperties": false, "required": [ "currency", "horizonYears", "loanAmount", "monthlyPayment", "upfrontCash", "priceToRentRatio", "breakEvenYear", "staysAhead", "atHorizon", "firstYearMonthly", "totals", "scenarios", "sensitivity", "years" ], "properties": { "currency": { "type": "string" }, "horizonYears": { "type": "integer" }, "loanAmount": { "type": "number" }, "monthlyPayment": { "type": "number" }, "upfrontCash": { "type": "number" }, "priceToRentRatio": { "type": [ "number", "null" ] }, "breakEvenYear": { "type": [ "integer", "null" ] }, "staysAhead": { "type": "boolean" }, "atHorizon": { "type": "object", "additionalProperties": false, "required": [ "buyerNetWorth", "renterNetWorth", "advantageOfBuying" ], "properties": { "buyerNetWorth": { "type": "number" }, "renterNetWorth": { "type": "number" }, "advantageOfBuying": { "type": "number" } } }, "firstYearMonthly": { "type": "object", "additionalProperties": false, "required": [ "ownerCash", "ownerUnrecoverable", "renterCash" ], "properties": { "ownerCash": { "type": "number" }, "ownerUnrecoverable": { "type": "number" }, "renterCash": { "type": "number" } } }, "totals": { "type": "object", "additionalProperties": false, "required": [ "interest", "ownerCosts", "renterCosts", "saleCosts", "saleTax" ], "properties": { "interest": { "type": "number" }, "ownerCosts": { "type": "number" }, "renterCosts": { "type": "number" }, "saleCosts": { "type": "number" }, "saleTax": { "type": "number" } } }, "scenarios": { "type": "array", "items": { "type": "object", "additionalProperties": false, "required": [ "label", "appreciationPercent", "breakEvenYear", "staysAhead", "advantageAtHorizon" ], "properties": { "label": { "type": "string" }, "appreciationPercent": { "type": "number" }, "breakEvenYear": { "type": [ "integer", "null" ] }, "staysAhead": { "type": "boolean" }, "advantageAtHorizon": { "type": "number" } } } }, "sensitivity": { "type": "object", "additionalProperties": false, "required": [ "appreciationPercent", "investmentReturnPercent", "breakEvenYear" ], "properties": { "appreciationPercent": { "type": "array", "items": { "type": "number" } }, "investmentReturnPercent": { "type": "array", "items": { "type": "number" } }, "breakEvenYear": { "type": "array", "items": { "type": "array", "items": { "type": [ "integer", "null" ] } } } } }, "years": { "type": "array", "items": { "type": "object", "additionalProperties": false, "required": [ "year", "homeValue", "mortgageBalance", "ownerCosts", "interestPaid", "renterCosts", "buyerPortfolio", "renterPortfolio", "buyerNetWorth", "renterNetWorth", "advantage" ], "properties": { "year": { "type": "integer" }, "homeValue": { "type": "number" }, "mortgageBalance": { "type": "number" }, "ownerCosts": { "type": "number" }, "interestPaid": { "type": "number" }, "renterCosts": { "type": "number" }, "buyerPortfolio": { "type": "number" }, "renterPortfolio": { "type": "number" }, "buyerNetWorth": { "type": "number" }, "renterNetWorth": { "type": "number" }, "advantage": { "type": "number" } } } } } }, "units": { "type": "object", "additionalProperties": { "type": "string" } }, "assumptions": { "type": "array", "items": { "type": "string" } }, "sources": { "type": "array", "items": { "type": "object", "additionalProperties": false, "required": [ "title" ], "properties": { "title": { "type": "string" }, "url": { "type": "string" }, "publisher": { "type": "string" }, "retrievedAt": { "type": "string" }, "note": { "type": "string" } } } }, "warnings": { "type": "array", "items": { "type": "string" } }, "presentation": { "type": "array", "items": { "type": "object" }, "description": "Display blocks used by the human page (same data)." }, "links": { "type": "object", "additionalProperties": false, "required": [ "human", "api", "openapi", "methodology" ], "properties": { "human": { "type": "string", "format": "uri" }, "api": { "type": "string", "format": "uri" }, "openapi": { "type": "string", "format": "uri" }, "methodology": { "type": "string", "format": "uri" } } } } } } } }