UK Property Strategy Simulator 🇬🇧

Current Status: Active Development. See Roadmap & Tracks.

A professional-grade Rent vs. Buy calculator designed for the UK market. Unlike simple mortgage calculators, this tool simulates 40-year wealth projections considering complex tax rules, inflation, and opportunity costs.

🌍 For Users

👉 Try the Live App: https://itsmuaaz.github.io/rent-vs-buy/

Why this tool?

Most calculators just compare "Mortgage vs Rent". This tool goes deeper, modeling your entire financial life over 40 years to answer: * "Am I better off buying a home, or renting and investing the difference in the S&P 500?"*

Key Features

• Multi-Strategy Comparison: Compare 6 different financial paths simultaneously:
  • Rent & Invest
  • Buy Home
  • Buy Home + Lodger (Tax-free income)
  • Buy-to-Let (Personal Name)
  • Buy-to-Let (Limited Company)
  • Home + Buy-to-Let Combo
• Transparency Mode (Audit): Click the "Audit" button to see exactly how your numbers were calculated (Stamp Duty, Amortization, Tax) with step-by-step proofs.
• True Valuation Modes: Toggle between Gross Equity (Paper Wealth) and Liquid Cash (What you'd actually have if you sold everything today, after paying all taxes and fees).
• Advanced UK Tax Engine: Accurately models:
  • Stamp Duty Land Tax (SDLT) including Additional Dwelling Supplement.
  • Section 24 (Mortgage Interest Relief restrictions).
  • Capital Gains Tax & Dividend Tax.
  • Corporation Tax for Ltd Companies.
• Stress Testing: One-click simulation of market crashes (Stock Market -30% or Property Market -15%) to see if your strategy survives a recession.

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💻 For Developers

Tech Stack

• Core Logic: Pure JavaScript (engine.js). No DOM dependencies. 100% unit-tested.
• Audit Layer: audit.js provides the Logic Dictionary for transparency.
• UI Framework: Alpine.js for reactive state management.
• Styling: TailwindCSS (CDN for zero-build setup).
• Visualization: Chart.js.
• Testing: Jest for logic verification + Husky for pre-commit hooks.
• Type Safety: TypeScript (JSDoc) + npm run typecheck.

Architecture

The project follows a strict "Engine vs. Dashboard" separation of concerns:

1. engine.js: The source of truth. Contains all financial formulas and simulation loops. It is stateless and platform-agnostic (runs in Browser and Node.js).
2. app.js: The controller. Connects the UI inputs to the Engine and handles Chart.js updates.
3. index.html: The view.

Local Development

1. Clone the repository.
2. Run npm install to setup testing tools.
3. Open index.html in your browser (no build step required).
4. Run npm test before committing to ensure calculation integrity.
5. Run npm run typecheck to verify JSDoc types.

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📱 Did you know?

This entire project was developed on Android using Termux. It serves as a demonstration of what's possible with a powerful terminal environment on mobile.

If you are interested in turning your Android device into a development machine, check out my other project: 👉 termux-bootstrap