A cost centre is a part of the business you want to see costs for on its own — a department, a site, a team, a project. Allocation is the business of getting each cost to the right one.
Direct costs are easy: an invoice for materials on the Salford job belongs to the Salford job. The difficulty is everything shared. Rent, insurance, the phone bill, the accountant. Those have to be split across cost centres on some basis, and the basis is a judgement.
Why bother
The honest answer is that it's only worth doing if somebody acts on the result.
Accountability. A budget holder can only be accountable for costs they can see and influence.
Pricing and profitability. If you want to know whether a job or a product line made money, its share of the overheads has to reach it.
Deciding things. Whether to close a site, take on a contract, or change a service — these need costs attributed, not pooled.
If none of those apply, allocation is bookkeeping for its own sake. Plenty of small businesses correctly do very little of it.
Picking a basis
The split has to rest on something. Roughly in order of how well they hold up to being questioned:
Actual usage. Metered electricity per site, licences per user, hours booked per job. Strongest, because it isn't an estimate.
A physical measure. Floor area for rent, rates and cleaning. Headcount for per-person services. Vehicle count for fleet insurance. Defensible and stable.
Time. Anything spanning a period — an annual premium across twelve months, which is a prepayment doing the same job.
Revenue or activity share. Weaker, because it moves for reasons unrelated to the cost centre. Fine for genuine overheads; poor for anything a budget holder is accountable for, since their allocation changes when someone else's sales do.
A fixed percentage agreed once. The most common in practice and perfectly reasonable — as long as someone can say where it came from and it gets reviewed occasionally.
The choice matters less than writing it down. An allocation nobody can explain is one that gets argued about every time it appears.
Where it goes wrong
Splits that go stale. The percentages were right when set, and the business moved. Headcount changed, a site closed, the project finished. Nothing about a stale split looks wrong, which is why it can misallocate for years unnoticed.
Allocating costs nobody controls. Charging a department for something they can't influence produces resentment and no better decisions. If the budget holder can't change it, consider leaving it in a central pool.
Splitting immaterial amounts. A £40 invoice split three ways is three coding decisions and a rounding argument for £13 of information.
Too many cost centres. Every one is a decision on every invoice. If you'd never look at one on its own, it doesn't need to exist.
Encoding two dimensions in one field. "Travel — Manchester" as an account is a chart of accounts doing a cost centre's job. Use the account for what it was and the cost centre for where — most accounting packages support both, and Xero allows two tracking categories.
A workable setup
Keep the list short. One per thing someone is actually accountable for.
Set defaults per supplier where a supplier only ever serves one cost centre. Most of the allocation then happens without a decision.
Use recurring templates for stable splits, so a monthly bill divided the same way for three years isn't thirty-six identical judgements.
Ask suppliers to itemise. Underrated. Many will break a bill down by site or department if asked, which turns a monthly estimate into a monthly transcription — and the basis becomes their usage data rather than your guess.
Review annually. Take the recurring splits and ask whether the basis still holds. Five minutes each, once a year, and they stay defensible.
The test
If someone asks why their department carries £800 of a £3,000 invoice, you should be able to answer in one sentence and point at where the rule is written.
If the answer is "that's how it's always been split", the allocation is a habit rather than a method — which is fine until someone makes a decision on it.
Cribble reads invoices to the line item rather than just the header, so a multi-line invoice arrives with the detail intact and ready to allocate. Coding you apply is remembered per supplier, so a split set once stops being a monthly decision.