Testing Claude Code With Real Domain Knowledge

I’ve had a personal interest in property investment for a while, and had done these evaluations manually before. This tool was partly a test of what Claude Code could do when fed real domain knowledge, built from scratch with AI by explaining the relevant factors, rather than porting an existing spreadsheet like I did with the business case tool.

The core of the property investment calculator is a side-by-side comparison: the same property, valued as a private purchase and as a purchase through an AB, a Swedish limited company. Swedish rules differ meaningfully between the two on loan limits and tax treatment, so the tool lets you check whether a specific property is worth it, and under which structure, without rebuilding the calculation from scratch each time.

It serves two purposes. It’s genuinely useful if you’re evaluating a rental property purchase in Sweden. And it’s a demonstration of what’s possible building a domain-specific tool with Claude Code from your own know-how, so others don’t have to build the underlying model themselves.

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