Most important thing when renovating
What is the best way to improve the value in apartments or houses that works no matter where you are ?
Updating but not over updating . . . . see what the area is expecting and target that for your rennovation. Finding a property that is stuck in time that an update will boost value and marketability is key. But not spending money for upgrades that are out of place for the focused market it is in will not add the value people often think it would. We recently did a flip where we used marble looking countertops and even the appriaser said for the area, that was a great choice . . . $200 verus $4,000. The value went way up with what we did but in talking to agents and teh appriaser, going with higher end finishes would not have gained us what we spent.
There's no one thing that works in every situation. That's probably the most important thing. Recognizing that every deal will be different and optimizing profit on every deal will require a different business plan and strategy.
I recommend reading through a cost vs. value report like this one: https://www.remodeling.hw.net/...
Jeff Thorman from Home Renovision DIY also made a great video on this topic a couple years back. The gist is that exterior updates generally have the greatest ROI because they boost curb appeal and usually cost less than complex, multi-phase projects (e.g. kitchen and bath remodels). I'm surprised painting the exterior of the house isn't on the list I linked above, since that's a really good place to start (assuming it needs a new paint job).
Also, echoing what Andy said, don't over-update. You don't want your house to be way nicer than all of the other houses in the neighborhood, because then all those comps will drag your appraisal down.