8 Proven Steps to Take Over a Legacy Codebase (Without Losing Your Mind)

13 Nov 2025 #coding

Like it or not, you can’t escape from legacy code.

I know how it feels to inherit a codebase like that. The “what I’m supposed to do now” feeling. While I’m writing this, my latest gig is yet another legacy code.

I’m not alone. Recently I found this question on Reddit, a fellow coder who received a codebase with only a handoff from a coworker.

I’m about to take over an .NET Core + SQL Server + Knockout.js/Angular project at my company. The issue: There’s zero documentation. I’ll only get a short handover from a teammate…

My main questions are:

• For legacy .NET projects, what’s your process to get up to speed fast?
• Should I start writing my own documentation (README, architecture notes, etc.) while I learn?
• Any tools/tips for mapping out the database + system structure quickly?
• From your experience, what do you focus on first: business logic, database schema, or the code itself?

I’d love to hear how you’ve handled taking over undocumented C# / .NET projects before. Thanks!

Having been in their shoes in every job I’ve had, here’s how I’d approach inheriting a legacy codebase, step by step:

#1. Understand it takes time.

It takes 6-12 months to know the ins and outs of a medium-to-large legacy codebase. Your mileage may vary. The point is: You don’t have to know every single small detail up front.

#2. Install and run the app.

Your first step should be to have your working environment running on your machine.

Install the tools you need: maybe a database engine, editor plugins, or scripts.

Also, this is a good opportunity to document those prerequisites in a README file and to open your first pull request.

#3. List the main components.

Don’t jump to files and methods.

Instead, get to know the overall architecture of the app:

• Is that a monolith? Does it use microservices?
• How many databases does it use?
• Does it call external APIs or services?

You can go as simple or as fancy as you want. Maybe that’s a sketch on a whiteboard or a version-controlled compiled diagram. I tend to go simple with pen and paper.

#4. Learn the building blocks.

Navigate the source code of the main solution or entrypoint.

Try to answer:

• What is the folder structure?
• How is the code organized? Per project? Per domain?
• How does it access the database? Stored procedures? An ORM? Plain SQL queries?
• How does it achieve common tasks like logging, REST calls, etc?

#5. Look for the most frequently changed files.

This is a trick I learned from Your Code Like a Crime Scene by Adam Tornhill.

Those are the places that attract complexity. That’s where bugs tend to happen or where requirements aren’t clear. They’re like the most dangerous places in your code.

If you’re lucky and the code is under version control, use and tweak this Git command,

alias gmost='git log --pretty=format: --name-only | sort | uniq -c | sort -rg | head -10'

I probably found it on StackOverflow. Or use fancier code analysis tools instead.

#6. Find out the most frequently used features.

Get to know how the end users interact with the app and what the 20% of features they use 80% of the time.

You will find one single screen they use daily, or just a few buttons out of a dozen. “Do one thing and do it well” is often a dream in enterprise software.

#7. Find out the module or component that gets more bug reports.

This is the spot to focus on first or to get early victories.

It doesn’t have to be the same one as the most used component or feature. Maybe it’s a report that runs once a month or a page that takes forever to finish. Or whatever.

Ask users about common pains. Maybe some ex-coworker always mentioned a fix. “Oh, every time that thing broke, Bob mentioned he had to…” You’re like a detective looking for clues.

#8. List key tables and parametrization tables.

Don’t try to understand every detail. Start with the overall design.

Is it one database per client in the same server? Or separate schemas per client?

Then try to generate a diagram from your database schema.

Most likely, you’ll find one or two key tables. Those are the “Reservations,” “Invoices,” or “Orders” tables. They tend to attract more references.

And you’ll also find parametrization tables. Those are tables with only a few records that power the behavior of the system. They’re good candidates for caching.

Parting thought

With those steps, you’ll get a clear big-picture of the codebase. Start broad, then move on to the details when you need it. That’s far better than trying to understand every single class or method up front.

Working with legacy code isn’t glamorous, but it’s the reality of our work. Most of the time, we’re maintaining and modernizing live systems, not building new ones from scratch.

That’s why I dedicate a chapter to legacy code in my book, Street-Smart Coding: 30 Ways to Get Better at Coding Without Losing Your Mind. Junior me thought I’d only start new projects with the latest tools. Wrong!

Grab your copy of Street-Smart Coding here. It’s the roadmap I wish I had moving from junior to senior and the one I hope helps you too.