Spec-Driven Development Cuts Digital Savings Build-Cost by 60% for UK Financial Services Group
60% lower build cost than the original delivery estimate
4-month delivery vs. conventional 12–18 month estimate
Capability retained with 12 customer colleagues upskilled in modern AI engineering practices
Background
Our client is a UK mutual financial services group with a savings book of around £70 billion, serving several million members.
Savings is a core product, but its digital proposition was lacking. Around 6 in 10 customers who started the savings journey did not complete it, while there was no way to open a core savings product end-to-end on the web.
The Group needed to improve the customer journey, but also needed to deliver at pace and within a cost it could justify. Conventional delivery had been estimated at millions of pounds over 12–18 months, putting it at odds with existing programme deadlines.
The Group wanted to test whether AI-native engineering could deliver a production-grade, regulated customer journey faster and at materially lower cost, while developing capability that could be reused across future propositions.
This savings journey became the first live proof point.
Solution
The engagement was delivered as an organisational test using AI for faster, lower-cost digital delivery, combining production delivery with capability building inside the client's environment.
AI-enabled delivery: Spec-driven development, using the client's AWS Kiro tooling within its existing CI/CD processes, made specifications the contract between people and AI agents. Pre-defined quality thresholds, audit trails and human review gates enabled AI to be used at production scale without compromising governance.
Production at scale: WeBuild-AI delivered 400+ screens across the end-to-end savings journey, including product selection, regulatory requirements and account opening. A new desktop-to-mobile authentication pattern removed the need for a separate mobile development programme, further reducing delivery complexity.
Built for control and change: Running in the client's own cloud tenancy, with audit, logging, telemetry and analytics embedded from day one, the solution met the requirements of a regulated environment. Its decoupled architecture also allowed delivery to progress in parallel with the wider core banking transformation, rather than waiting for it to complete.
Impact
The completed delivery was 60% below the original quoted price for end-to-end delivery, demonstrating a materially different cost model for digital change.
The digital experience and supporting services were delivered in four months against a conventional 12–18 month delivery expectation, reducing time to customer value and allowing the Group to redeploy investment into their wider transformation goals.
The UK savings market is approximately £1.98 trillion. The engagement creates the digital capability required to compete for an increasing share of that opportunity. Assuming a conservative lift of just 2 additionally retained customers secures ~£224 million in new deposits, driving a +0.011 percentage point increase in total UK market share.
For a regulated business with £70.1 billion of savings, regulatory compliance, auditability and resilience are critical to protecting existing value. Quality thresholds, testing, human review, audit and telemetry were built into the delivery approach rather than added afterwards.
12 client colleagues were upskilled in modern AI engineering methods, including spec-driven development, AI tooling and AI-assisted workflows. More than 60 delivery controls were captured in a reusable playbook, leaving the capability inside the organisation rather than with an external delivery team.
Why WeBuild-AI
WeBuild-AI was chosen for our proven credentials in embedding and scaling spec driven development against regulated products/environments and our focus on enabling the customer through the course of delivery. Our engineers deploy into the client's own squads and tooling rather than advising from the outside, enabling the client’s team to deeply upskill and learn the methods and approaches. Everything was provisioned in the client's cloud and tenancy under controls they already trusted, and capability transferred as we built rather than arriving as a handover at the end.
