[tasuki]

In my experience, writing code that tries to be extensible also results in extensive refactoring when new functionality is introduced.

My advice would be to make the code easy to understand and relatively un-entangled. Those make the inevitable refactorings easier.

[ozim]

I would say don’t try to make code extensible - try to make code replaceable.

If it is not complex it is easy to rewrite and replace. Some things need to be complex but trying to make things extensible is making it needlessly complex quite often.