New app boosts customer ratings 200% for high street bank

The Zühlke team enabled a major global bank to boost its customer ratings by 200% with a transformed app and stronger API infrastructure.


  • ~6 million customers migrated to banking app

  • New app secures 200% higher customer rating

  • Provides superior functionality and UX, powered by a modern, secure API framework

The challenge

Our multinational bank client provides a range of retail banking and wealth management products to around 14 million customers in the UK.

Its UK app for iOS and Android was created to help customers manage their accounts in a simple and convenient way.

But the app was failing to deliver on this promise. Technical bugs, performance issues and a poor customer experience hampered customer satisfaction and app store ratings.

An example of one of these issues was it took a sluggish 30 seconds for users to log into their account.

These issues stemmed from problems in the mobile app build and supporting API infrastructure, and bugs were making their way into new releases, despite the lengthy 12-week process to get them live.

The bank therefore identified two ways to improve UK customer satisfaction and elevate app store ratings:

  • Deliver a new, high performance mobile banking app with a superior user experience.
  • Improve the supporting API infrastructure to solve critical performance issues, including the slow log-in.

What we did

With our proven expertise in mobile banking and regulated apps, Zühlke was an ideal choice of partner to help the bank deliver its core workstreams.

We kicked things off with a detailed discovery phase led by our multidisciplinary team. Our UX designers got to work quickly on user research and testing to better understand the problem space, user needs, and business requirements we’d need to address in an MVP app.

Meanwhile our engineers and technical architects scrutinised the legacy code, infrastructure, and engineering and delivery processes to investigate the technical issues and identify dependencies.

Working closely with the product head and delivery manager, we built the UK mobile banking app and supporting infrastructure, enabled a secure and phased migration of users to the new app, and provided essential technical support.

Here’s a look at some of the key workstreams.

Building the mobile banking app

We built an accessible mobile app that complied with the Web Content Accessibility Guidelines (WCAG).

The focus was on a stable, secure, digital product that met legal and compliance needs. This included creating documentation at scale to provide a clear and simple audit trail for the financially regulated and audited app and using native iOS and Android to ensure security levels were achieved.

Threat modelling was required on particular features to identify how a hacker would potentially look to exploit it, and ensure the protection of customer data.

Each bit of functionality was rigorously tested for quality assurance, and user tested to ensure value delivery to customers.


Improving banking architecture

Our technical architects focused on strengthening backend APIs and infrastructure security.

We optimised and improved the API pipeline and implemented two-factor authentication on money transfers and payments to increase security.

Thanks to this essential backend work, we were able to cut the login time from 30 seconds to just one second.

technical architect

Building knowledge and capability

Our team gained in-depth insights into how the entire end-to-end stack worked – from authentication to authorisation.

Much of this knowledge was lacking in the business, so we ran API surgeries with other teams to transfer our learnings.

building knowledge

Optimising processes

We worked with backend IT to bring stability, clarity, and efficiencies to the test environment, build pipeline, and dependency management process.


Providing technical and service support

Customer support staff needed to understand how app user flows worked, so that they were equipped to support customers as they encountered any issues across these user journeys.

Meanwhile, legal and compliance teams needed to understand what failures at different stages in these user journeys would mean in terms of access to account and customer information.

Our team were instrumental in creating call centre scripts, legal and support team documentation, and 24/7 support rotas. We trained and equipped our engineers and architects to provide third-line support, join support calls, and report effectively on support issues.

tech support

Migrating users to the new app

A key challenge was how to migrate users securely to the new app and handle the new biometrics authentication and password requirements.

We needed a highly secure way to move information, but in a way that ensured we couldn’t learn the specifics about this information. This was essential for protecting customer records and preventing security breaches.

To solve this challenge, we created a framework that was embedded in both the old and the new app. The framework guided the customers through the migration process and securely transferred their credentials between the apps, without it ever leaving the device.

We also developed an isolated backend service that could control the rate of migration and, in case of an incident, act as a failsafe.

app user

The results

We delivered a high performance mobile banking app and supporting infrastructure, migrating around 6 million customers securely.

The new app achieved a 200% higher customer rating on The App Store, thanks to its superior functionality and UX, powered by a modern and secure API framework.

Our work on the UK app helped the bank go on to develop an integrated global app, which used many of the components, UX elements, and APIs we created.