Moving a building society's loan book — with its history intact
A £1.2bn society replaces its legacy core with Mambu. The balances are easy. The decade of arrears history is the job.
The situation
A regional building society, ~£1.2bn balance sheet, replaces its end-of-life core (a Phoebus-generation system) with Mambu. The programme has a vendor, an SI for the platform build, and a hole where the data workstream should be: 38,000 live mortgage and savings accounts, plus ten years of payment, arrears and forbearance history that regulatory reporting and Consumer Duty evidence both depend on.
Week 0–2: Migration Readiness Assessment (£15k fixed)
Both estates profiled. The critical artefact is the history mapping decision set: which events move into Mambu's model, which archive into a queryable store with lineage, and how the old core's arrears status taxonomy translates — event by event — into the new one. Every decision written, versioned, and signed by the society's head of credit risk, not left in a contractor's head.
Week 3–10: Migration Delivery (fixed per the assessment; typical band £60–120k)
Live positions and history migrate in separate, separately-reconciled streams. Dry runs prove both: balances to the penny, and the harder test — yesterday's arrears status equals today's arrears status for every account, which is what catches subtle mapping errors. Cutover is phased: savings first, mortgages the following month, each with its own evidence pack and each reversible until go/no-go.
The evidence pack
Account-level reconciliation across both streams; balance totals agreed to the penny per product; arrears-status equivalence proven account by account; the archive store verified queryable with ten sampled regulatory-report reconstructions. Signed, filed, and — when the next FCA data request arrives — load-bearing.
What could go wrong — and how it's handled
Forbearance arrangements are where core migrations quietly fail: bespoke arrangements recorded as free-text notes in the old system don't map to anyone's data model. The assessment counts them on day three (they're always findable by pattern), and the society decides the handling policy in week two — not during cutover weekend.
Recognise your situation? Tell us the source system, the target, and the deadline — we'll tell you within 48 hours whether we can hit it and what the Assessment will cost.
freddie@godwit.uk