Source: https://guthlabs.ai/docs/instruments/rent-vs-buy-home-comparator/implementation/

instrument implementation / implementation-contract

# Rent vs Buy (Home) Comparator implementation

A source-bound implementation contract for Rent vs Buy (Home) Comparator; deployment and executable qualification remain separate.

Status: **VERIFIED_CLEAN**Lifecycle: **implementation-contract**

## Binding contract

This record describes what an executable surface must bind and validate. It does not claim that a public calculator, package, or production route is deployed.

**Record owner**: Guth Instruments

**Lifecycle**: verified-clean

**Evidence ID**: t5-154

**Effect**: Computation or source lookup only

## Input and validation requirements

The zero mortgage-rate case produces 0/0 under the ordinary mortgage-payment formula, and the zero-ratio rent case requires a separate geometric-series branch. The specification also leaves mortgage term, time indexing, and appreciation sign conventions insufficiently defined.

## Required corrections

No binding erratum is attached to this unique-name record. Implementers must still preserve the declared units, domain, and failure behavior.

## Inspectable interface fixture

The sample below is evidence for interface and validation tests, not a production implementation.

````
{
  "inputs": {
    "home_price": 300000,
    "down": 60000,
    "horizon_years": 3,
    "annual_mortgage_payment": 16000,
    "annual_tax": 3600,
    "annual_insurance": 1200,
    "annual_maintenance": 3000,
    "principal_paid_by_year": [
      6000,
      6200,
      6400
    ],
    "appreciation_PV_by_year": [
      3000,
      2890,
      2780
    ],
    "annual_rent": 24000,
    "rent_growth": 0.02,
    "discount_rate": 0.04
  },
  "expected_outputs": {
    "buy_cost": 104130,
    "rent_cost": 69266.1,
    "breakeven_year": null
  },
  "working": "Buy year 1 = 16000 + 3600 + 1200 + 3000 - 6000 - 3000 = 14800. Buy year 2 = 16000 + 3600 + 1200 + 3000 - 6200 - 2890 = 14710. Buy year 3 = 16000 + 3600 + 1200 + 3000 - 6400 - 2780 = 14620. Buy total = 60000 + 14800 + 14710 + 14620 = 104130. Rent year 1 PV = 24000 x 1.02 / 1.04 = 23538.46. Rent year 2 PV = 24000 x 1.02^2 / 1.04^2 = 23085.80. Rent year 3 PV = 24000 x 1.02^3 / 1.04^3 = 22641.84. Rent total = 23538.46 + 23085.80 + 22641.84 = 69266.10. Breakeven is null because the specification does not define its comparison rule or sale treatment."
}
````

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/rent-vs-buy-home-comparator/>)[02**Mechanism**](<https://guthlabs.ai/docs/instruments/rent-vs-buy-home-comparator/mechanism/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/rent-vs-buy-home-comparator/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche5/tranche5-results.json`