Architectural Fee Calculator Development

I need a robust yet easy-to-use system that instantly generates my architectural and structural-engineering fees once I enter a plot size and choose a building type. The calculator must cover both residential and commercial projects and drill down to the following categories:

• Residential: single-family homes, multi-family homes, apartments
• Commercial: offices, retail stores

The end result can be delivered as a well-structured Excel/Google Sheets model, a lightweight web tool, or a plug-in for an existing CRM—whichever you feel best suits accuracy, transparency, and future scaling. I already have rough percentage ranges per square metre for each category; what I need from you is the logic, the clean formulas, and a user-friendly interface that lets me tweak coefficients (site slope, number of storeys, design complexity, local permits, etc.) without breaking anything.

Please ensure:
1. Clear, commented formulas or code so I can audit and adjust rates later.
2. Separate outputs for architectural fees, structural fees, and a combined total.
3. A short walkthrough (video or screenshare) demonstrating the tool on at least one residential and one commercial scenario.

If you’ve built similar cost-calculation tools—especially for AEC—let me know; that experience will weigh heavily in my selection.