From Spreadsheets to a Monitoring Tool: 200 Clients

Practices tend to outgrow their deadline spreadsheet somewhere between 40 and 60 clients. By 200, the sheet is actively dangerous, and migrating off it without losing dates needs a method, not a weekend.

Why the spreadsheet eventually breaks

The classic symptoms: 'deadline tracker v17 FINAL (Sarah's copy).xlsx' sitting alongside the master, date columns silently reformatted as text after a CSV round-trip, conditional-formatting rules that overlap and contradict each other, and a quiet weekly habit of opening Companies House to double-check whether the year-end on the sheet is still the year-end on the public record.

Under that surface the structural failures are worse. Year-ends move and no one updates the sheet: a subsidiary acquisition shortens the AP and the CT600 date is wrong for six months before anyone notices. New clients added mid-onboarding sit in someone's inbox, not the tracker. Strike-off and dissolution are invisible, so you chase accounts from a company that no longer exists. Outlook reminders belong to one person, who then leaves.

To be fair: for a sole practitioner under 15 clients, Excel is genuinely fine. The breakdown is a function of headcount and number of editors, not a moral failing of spreadsheets.

Auditing what your current sheet actually tracks

Before exporting anything, list what the sheet thinks it knows: company number, year-end, accounts deadline, CT deadline, confirmation statement date, payroll cycle, VAT quarter, engagement status. Then list what it doesn't track but should: PSC changes, registered-office changes, strike-off notices, name changes. The gap between those two lists is the case for a dedicated tool, and the migration scope.

Exporting and cleaning the client list

Don't seed the new system from the spreadsheet. The spreadsheet is the problem. Pull a fresh list directly from Companies House, either via the bulk product data or by company-number lookup, and reconcile against your engagement letters. Anything in the spreadsheet but not in your current engagement list is a candidate for removal. Anything in your engagement list but not in the spreadsheet is the gap that's been costing you.

Mapping companies to Companies House numbers

Every client needs its eight-digit CRN. Names won't do. 'Smith Holdings Ltd' returns dozens of hits and homonyms move around. If you don't already capture CRNs systematically, this is the slowest part of the migration. Budget an hour per 30 clients for a clean lookup, more if any are LLPs or Scottish-prefix (SC).

Reconciling deadlines on day one

Once loaded, compare each deadline in the tool against the spreadsheet. Mismatches are the discovery, not the noise. Every mismatch is a date the sheet had wrong. Where there's a difference, the public Companies House record wins; that's the date the regulator will use. For HMRC obligations, your year-end drives everything, so the reconciliation is mechanical once year-ends are correct.

For the bulk-check workflow itself we've written a separate guide on how to check Companies House filing deadlines in bulk.

Running both systems in parallel

Run the spreadsheet and the new tool side by side for at least 30 days. Two reasons. First, you'll find dates the sheet had wrong, and without parallel running you'll never know whether the tool 'missed' something or just corrected it. Second, the team needs to develop trust before they stop double-checking. Skip the parallel run and you'll get a quiet drift back to the sheet within a fortnight.

Decommissioning the spreadsheet safely

When you do retire it: archive a final dated copy somewhere read-only, communicate the cutover date in writing, and remove edit access from everyone. Don't leave the sheet on the shared drive 'just in case'. Within three months someone will have updated only the sheet and not the tool.

One last thing that gets overlooked. The spreadsheet almost certainly contains personal data: director names, dates of birth, sometimes NI numbers. Under Art.32 UK GDPR, an unencrypted .xlsx forwarded to a personal email is exactly the kind of breach vector the ICO has fined firms for. The migration is a good moment to delete every old copy floating around the team's inboxes. For more on this side of the question see our note on Companies House monitoring and GDPR.

For a broader view of what else is available before you commit to a tool, our comparison of Companies House reminder tools covers the landscape honestly, including the cases where staying on Excel is the right call.