Why one developer quotes 3 days and another quotes 3 weeks

Short answer: the difference between a three day quote and a three week quote is usually that one developer is proposing a theme app extension and the other a full custom app. One question separates them: does this need to remember anything, or talk to anything outside Shopify? No to both means an extension, which is days, no hosting and no monthly cost. Yes to either means an app.

A merchant asks for something simple. A size guide that opens from the product page, styled to match the store, editable without a developer.

Two developers quote it. One says three days. The other says three weeks. Both are describing real work, and the difference is not skill or margin. It is that they are proposing different things, and usually nobody explains that to the merchant.

Here is the difference, in terms that affect your invoice and your ongoing costs.

Three ways to add functionality

A theme app extension. Code that runs in your storefront, installed through Shopify's own infrastructure rather than pasted into your theme. The merchant drags it into place in the theme editor like any other block.

A custom app. A full application, built for your store only, with its own admin interface, its own hosting, and its own database if it needs one.

A public app. The same thing, but listed on the App Store for anyone to install. Only relevant if you intend to sell it.

Most merchants who say "I need an app built" want the first one, and pay for the second.

When is a theme app extension enough?

It is the right answer when the thing lives entirely in the storefront and does not need to remember anything between visits.

Cases we see constantly:

• A size guide or fit finder
• A delivery date estimator
• A custom product badge driven by tags or metafields
• A stock countdown or low-stock message
• A sticky add-to-cart bar on mobile
• A comparison table
• Reviews display, where reviews come from somewhere else

None of those need a server. They read data Shopify already holds, render it, and that is the job.

What it costs you: days rather than weeks. No hosting bill. No monthly fee. No maintenance beyond occasional Shopify updates. Faster than the old approach because it loads through Shopify's infrastructure instead of injecting a script tag into your theme.

What you get: the merchant can add, configure and remove it in the theme editor, without a developer. That last part is the difference between a site your team runs and one they phone about.

When do you actually need a custom app?

An app becomes necessary when something has to happen outside the storefront.

It stores its own data. A quote builder that saves quotes. A loyalty scheme with balances. Anything with records Shopify does not hold.

It talks to another system. Your ERP, your 3PL, an accounting package. That needs a server that can authenticate, retry, and log.

It runs on its own schedule. A nightly sync, a report, a job that runs whether or not anyone is on the site.

It reacts to Shopify events. Webhooks on order creation or fulfilment need somewhere to arrive.

Staff need an interface. If someone on your team needs a screen inside Shopify admin to manage something, that is an app.

What it costs you: weeks rather than days. Hosting, small but real. Ongoing maintenance, because you now own a running system. Two API versions a year to keep up with.

That is not an argument against it. It is an argument for knowing which one you are buying.

The question that separates them

Does this need to remember anything, or talk to anything outside Shopify?

No to both: theme app extension. Yes to either: custom app.

Almost every case we are asked about resolves in one question.

Where the money goes wrong

Being quoted an app for extension work. The most expensive version of this mistake. Weeks of work, hosting, and maintenance for a size guide. If you are quoted an app, ask directly: "could this be a theme app extension, and if not, why not?" A good developer will have a specific answer about storage or external systems. Vagueness is a signal.

Being quoted a public app for internal use. Public apps carry App Store review, ongoing policy compliance, and Shopify's revenue share. If you want a thing for your store, you want a custom app, and the public route is significantly more work for no benefit to you.

Pasting code into the theme instead. The old approach: a script tag dropped into theme.liquid. It works, and it costs you two things. It loads on every page whether or not it is used, and it disappears the next time you change or update your theme. Every theme update becomes an archaeology exercise in what custom code has to be carried across.

Buying an app subscription for extension-shaped work. $30 a month for a size guide is $1,080 over three years, plus a script on every page. A theme app extension is built once, owned, and free to run.

What should you ask before you sign?

"Is this an extension or an app, and why?" The reasoning tells you whether they have considered it.

"Where does the code live?" In the theme, in an extension, or on a server. Each has different consequences when you next change theme.

"What is the monthly cost after launch?" An extension should be zero. An app should be hosting only, and they should be able to name a figure.

"Will it survive a theme change?" An extension does; it is not part of your theme. Code pasted into a theme does not.

"Do I own it?" Yes should be the answer, with source handed over readable and commented, whichever route it is.

The rough shape of a decision

Storefront only, no memory, no outside systems: theme app extension. Days, no ongoing cost, and your team can position it themselves.

Data to store, systems to talk to, jobs to run, staff screens: custom app. Weeks, some hosting, real maintenance, and worth it when the requirement is genuine.

Selling it to other merchants: public app, and that is a product decision rather than a store decision.

The reason this is worth twenty minutes of your attention is that the gap between the two quotes is usually several thousand pounds, and the difference is often just that nobody asked the question.

---

We build both, and we tell merchants when the cheaper one is enough. App builds run 4 to 8 weeks at a fixed price; extensions are usually days. Either way the source is yours on completion, with no licensing.

Been quoted for an app and unsure it needs to be one? Book a free 30 minute call and describe what you want. You will get a straight answer on which it is. Or see our app development work. Related: build custom or install from the App Store and Shopify Functions instead of an app subscription.