When painting interior walls, how much old paint must you remove?

unless it is peeling you do not have to remove the paint, simply clean, apply tow coats of primer allow to dry and paint

if the existing paint is in good shape, no need to remove.

If the walls are cleaned, esp' with TSP, you don't have to remove anything.

https://www.diynetwork.com/how-to/skills-and-know-how/painting/how-to-prep-walls-before-painting

Never removed old paint. Prime then paint

Hiya Luke!

You don't have to remove any of the old paint, unless it's chipped and cracking. If that's the case, scrape off all loose paint and give the wall a skim coat with something like i've posted below. If the walls are just painted, with no paint chipping or cracking, you should use a paint and primer in one. They're available everywhere. I find the best to be Behr paint by Home Depot.

Why are you removing it? Is it peeling? If so then feather-sand the edges out and wipe clean with a dry cloth. Apply a good quality paint and primer all in one, I prefer Sherwin-Williams HGTV paint and primer all in one at Lowes, get the top quality formula in a finish suitable for the room its being applied too, (personally I never use flat). You pay more for it, but it covers in one coat saving you tons of time.

I only remove old paint if it's flaky or built up. Otherwise, I prime first if changing from dark to light.

You're very welcome!