Shopify Development UK: Themes, Custom Apps and Functions

When Shopify is the right choice

Shopify is a hosted platform: Shopify runs the servers, the checkout and the security patches, and you pay a subscription. For many retailers that is exactly right. There is no server to maintain, the checkout is fast and trusted, and the app store covers most common needs.

The trade-off is control. You cannot change everything, some features live behind paid apps, and certain checkout changes require Shopify Plus. Our job is to make Shopify do what your business needs within those rules, and to tell you honestly when a requirement would be easier on WooCommerce or a custom build.

After Scripts: Shopify Functions and checkout extensibility

Shopify Scripts, the Ruby snippets Plus merchants used for tiered discounts, shipping rules and payment method rules, were retired alongside checkout.liquid. Scripts could no longer be edited from April 2026 and stopped running on 30 June 2026. If your discounts or delivery rules quietly changed this summer, that is likely why.

The replacements are:

• Shopify Functions: small programs written in JavaScript or Rust, deployed through an app, that run inside Shopify's backend for discounts, delivery and payment customisation, cart and checkout validation, and more.
• Checkout UI extensions: components that add fields, banners, upsells or information to specific points in the checkout, rendered by Shopify so the checkout stays fast and secure.
• Theme app extensions: blocks an app adds to the storefront theme without anyone editing theme files.

We rebuild Script logic as Functions, test them against your real product and customer scenarios, and package them in a custom app you own.

Custom apps and the Admin GraphQL API

When a public app does not fit, or you are paying for several apps that each do a small job, a custom app built for your store is often better value over time. We build them on Shopify's current app stack using the Admin GraphQL API. Shopify has made GraphQL its primary API: its developer changelog states that new public apps submitted from 1 April 2025 must use GraphQL only, with the REST Admin API treated as legacy.

That shapes how we build. Queries request only the fields needed, large jobs such as catalogue exports use bulk operations, and our code respects Shopify's cost-based rate limits. Webhooks for orders, products and inventory are verified using their HMAC signature before we trust them, and processed through a queue so a burst of orders on a sale day does not overwhelm anything.

Typical custom app jobs: trade price lists, product feeds from a supplier, custom reports, order routing to a warehouse, or syncing invoices to Xero. See Shopify integration for more on back-office links.

Themes that editors can actually use

Online Store 2.0 themes are built from JSON templates and sections, so merchants can add, remove and reorder sections on any page from the theme editor. Many stores we see still run older, heavily edited themes where content is hard-coded into Liquid files and only a developer can change it.

We either customise a good existing theme or build sections tailored to your products: comparison tables, size guides, bundles, lookbooks, store finders. Each section has the settings editors need and no more. We keep custom code in a clear structure and in a Git repository connected through Shopify's GitHub integration, so changes are tracked and reversible.

Speed matters too. We keep app scripts off pages that do not need them, use responsive images and avoid piling apps onto the theme. Before adding any app, we ask whether a theme section or a small custom app would do the same job without another monthly fee and another script on every page.

Moving to Shopify, or starting fresh

If you are moving from WooCommerce, OpenCart, Magento or an older platform, we migrate products, customers and order history, map every old URL to its Shopify equivalent and set redirects so search rankings carry across. Shopify enforces its own URL patterns such as /products/ and /collections/, so redirects matter more than usual. See ecommerce migration.

Whether it is a new store, a Scripts replacement or a single custom app, we start with a free chat and give you a fixed-price quote for an agreed scope. Talk to us about your store.

What we deliver

• Theme builds and customisation on Online Store 2.0, with sections editors can use
• Custom or private apps for one store, built on the Admin GraphQL API
• Shopify Functions for discounts, delivery and payment customisation, and cart validation
• Checkout UI extensions for fields, messages and upsells within Shopify's rules
• Integrations with accounting, ERP, couriers and marketplaces using webhooks
• Migration onto Shopify from WooCommerce, OpenCart, Magento or another platform

How it works and what it costs

Every project gets a fixed-price quote after a free initial chat and a short scoping stage. You own the code and the data.

1. Free chat

   Tell us the problem in plain English: what you do now, what goes wrong and what "better" looks like. No charge, no obligation.

2. Scoping

   We map the processes, systems and data involved, agree what is in and out, and write it down so there are no surprises.

3. Fixed-price quote

   You get a fixed price for the agreed scope, or a phased plan for bigger builds, so you can start small and prove it works.

4. Build and test

   We build in short stages you can see and try, test against real data, then go live carefully with a rollback plan.

5. Hand over and look after

   You own the code and the data. We can host it, support it and keep improving it, or hand it to your own team.

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