Separate input, calculation and output
Three layers, visibly distinguished. Inputs are the only cells anyone types into and are formatted so that is obvious. Calculations are never mixed with presentation. Outputs reference the calculation layer and contain no logic. Almost every unmaintainable workbook we are asked to rescue has these three collapsed into one grid.
No hard-coded numbers inside formulas
A VAT rate written into forty formulas is forty edits and one guaranteed miss. Constants live in a named input cell, referenced everywhere. This is the single change that most reduces the cost of the next modification.
Structured references and named ranges
Tables and named ranges instead of A2:A5000. Ranges that grow with the data do not silently exclude last month's rows, and a formula that reads SUM(Sales[Amount]) tells the next person what it does.
Errors handled where they occur
IFERROR wrapped around an entire formula hides real problems. We handle the specific failure — a missing key, a divide by zero — and let genuinely unexpected errors surface where they can be seen and fixed.
Documentation in the file
A README sheet: what each input means, where source data comes from, what to change at year end, and who to contact. External documentation gets lost; a sheet inside the workbook travels with it.
You own the code
Unlocked modules, readable source, no licence checks. If you decide to take the work in-house or to another developer, nothing we have written stops you.