How to properly organize the structure of a constantly changing project?

Question

Development Methodologies

We have a web application that is used by a couple of hundred users of one closed organization. The application is cumbersome, with a vast amount of functionality, some of which we ourselves no longer remember.
The operation of the application is strongly tied to legislation, which changes from year to year.
It turns out this: they wrote some kind of fundamental class that uses another class, etc. and all this is combined into a module. And after a few months they say: the logic of work has changed, urgently redo it!
And you have to break the whole structure, often with crutches.
Question : Are there any methodologies for developing frequently changing code? So that it’s easy for us and for the customer “urgently”, as it always happens with him

Answer 1

It's funny. I re-read this old question. It's strange that none of the respondents mentioned agile development, SOLID, and development books and articles from Robert Cecil Martin.

Answer 2

Divide into small independent parts (such as microservices). Isolated-self-sufficient.
PS:
The skill of decomposition is the most important skill for a programmer developing complex systems.

Answer 3

In general, all these product owners and architects exist for this, don't they?
It is necessary, damn it, to keep the documentation in order, to develop it systematically, and not "from the kandachka". The approach needs to be changed, in principle

Answer 4

For an enterprise with extensive business logic, DDD is fine . It will be unrealistic to translate an already existing project with huge functionality and crutches at a time. You can try small iterations. The level of the team must also match, since the construction of such an architecture will definitely use a good half of the patterns described in the PoEAA book .