Taking over an existing app: code review, rewrite, or keep building?

Asking whether someone can take over an existing app is about as common as asking what a new app costs. The short answer is: often yes. But the right next step is rarely “just keep coding.” It is to understand whether the codebase is worth building on — or whether a rewrite will be cheaper and safer over time.

The longer answer is about three different jobs: code review, continued development, and rewrite. They sound similar, but they solve different problems and carry different risk. This guide helps you choose the right path before you switch agencies, inherit a freelancer’s code, or try to rescue an app that has become expensive to change.

At ScriptSector, the question Can you take over my existing app? comes up often. The answer always starts with a technical review — not with a promise that “we’ll fix it.”

Most takeovers are not about technology for its own sake. Something in the product or the partnership has broken:

• The previous developer or agency is no longer available
• Changes take too long or cost more than they return
• The app crashes, hangs, or fails new iOS and Android requirements
• You lack source code, documentation, or access to accounts
• You want new features, but nobody wants to touch the codebase
• You need a partner who can both maintain and develop further

If any of that sounds familiar, the next question is not “who can code fastest?” It is “what are we actually working with?”

A common trap is jumping straight to continued development. Then you pay for new features on top of problems you have not seen yet. The code review is what makes the next decision defensible.

decision: keep building, clean up first, or rewrite.

What we normally cover:

1. Access and ownership. Is there source code, Git history, design files, backend, API keys, and developer accounts? Who owns what? On new ScriptSector projects the principle is clear: you own source code, design, and documentation when the agreement is met. On a takeover, the same things must be clear before anyone writes new code.
2. Architecture. How is the app structured? Are there clear layers, or is everything tangled? Can backend, database, and admin panel evolve independently?
3. Stack and maintainability. Are dependencies current? Is there CI/CD? Can the app be built and shipped without tribal knowledge?
4. Security and integrations. Login, roles, BankID, payments, CRM, and business systems. Without a test environment or docs, every change gets slower.
5. Testability and risk. Are there tests? Crash reporting? Analytics? Without them, a “small fix” can break something else.
6. Commercial assessment. Is it financially sensible to keep developing this code, or will a rewrite cost less over time?

The output should be concrete: what works, what is risky, roughly what cleanup would take, and a recommended path.

Continued development fits when the review shows a reasonable structure, you own what you need, and new features can be added without every change becoming an experiment.

Typical signs:

• The codebase can be built, tested, and released
• The architecture is understandable to a new team
• Integrations are documented or at least stable
• You have a backlog with business value, not only fire drills

Then the working model is often the same as after a new launch: sprints, prioritisation, and ongoing maintenance. At ScriptSector, support and further development are part of how we work with app development.

Continued development does not mean ignoring debt. It means paying it down while you deliver value — for example by cleaning a module when you are already touching it.

A rewrite sounds expensive. Sometimes it is the cheapest option.

Consider a rewrite when several of these apply:

• Nobody can explain how critical parts work
• Small changes take weeks or create new bugs
• Dependencies are so old that every OS update becomes a crisis
• You lack tests, documentation, and a safe path to production
• The app was built for a goal that no longer applies (wrong platform, architecture, or product)
• The review shows that “saving” the code approaches the cost of a new first version

A rewrite does not have to mean throwing everything away on day one. Often the core flows are rebuilt first while older parts are phased out. The goal is a codebase you can maintain — not a perfect restart for style points.

If you still need a new first version, use the same logic as building new: cut the scope. An MVP Sprint of 6 weeks at a fixed price of SEK 60,000 excluding VAT can be right when you want to replace a broken app with a clear core flow, not recreate a ten-year feature list in one go. More complete business apps often land between SEK 200,000 and 500,000, while larger solutions can start from around SEK 500,000 — the same ranges as in How much does it cost to develop an app?

Takeovers go faster when the material is ready. Ideally collect:

• Git repo or latest source (iOS, Android, React Native, backend)
• Design files (Figma or equivalent)
• Environments: development, staging, production
• API docs and keys (shared securely)
• Apple Developer and Google Play accounts, plus permissions
• Hosting, databases, Firebase, or other infrastructure
• Crash and analytics tools
• Known backlog: bugs, requests, deadlines

Missing pieces are not a stop sign. They are information the review must account for — and often a warning about how much hidden debt exists.

1. Kickoff and access. You share code, accounts, and goals. What should the app do in three months? What must work tomorrow?
2. Code review. Architecture, risks, and ownership. You get a written assessment and recommendation.
3. Decision. Continue, clean first, rewrite parts, or combine.
4. Stabilisation. Before big features: get builds, release, monitoring, and critical bugs under control.
5. Roadmap in sprints. New features in 1–2 week sprints, with demos and clear priorities.
6. Maintenance. OS updates, dependencies, support, and ongoing product work.

This mirrors the same chain as in How long does it take to develop an app? time and cost follow clarity, not wishful thinking.

Demanding new features on day one. If nobody understands the codebase yet, every feature costs more than it should.

Assuming “we have the code” means it is usable. Without history, environments, and keys, you only have files.

Choosing a rewrite too early — or too late. Too early: you throw away something that could have been saved. Too late: you have already paid for several detours.

Forgetting business requirements. A technically elegant rewrite that lacks BankID, roles, or the integration the business runs on is not done.

Buying further development without maintenance. Apps go stale. Without a plan for updates and support, the same problems return. Ask the same kinds of questions as in 12 questions before you hire an app developer.

Can you take over an app without the original developer?
Yes, if you have source code and enough access to accounts and environments. If not, the first step is to reconstruct what you can — and to judge whether a rewrite is faster.

How long does a code review take?
It depends on size and how complete the material is. A focused app is quicker than a platform with several apps, an admin panel, and many integrations. The point is a review short enough to produce a decision, not a project of its own.

Are React Native apps easier to take over than native?
Sometimes, if the codebase is clean and documented. Stack matters less than structure, tests, and ownership. More on platform choice is in iOS and Android app development: technology choice.

Do we own the code after the takeover?
You should own the code and assets you pay to develop further. Clarify ownership in the agreement before work starts — for existing code and for what is built new.

We start with code and architecture, not with a sales pitch for a rewrite. Based on the review, we say plainly whether continued development is defensible, what needs cleaning first, or whether a new first version is the more honest option.

You get a clear recommendation, a sprint plan, and a team that can both save what works and build what is missing. You own the result. We stay for maintenance and further development once the app is used for real.

Want to take over an existing app, or unsure whether it is worth building on? Contact ScriptSector and we will walk through the codebase, the risks, and which path makes sense before you invest more.