In my dream I seem to come home after being gone a long time. My flat mate has let a room to a woman (we are both men, and in our actual flat we don't have any extra rooms, but in the dream the flat is bigger, may even be a house). The woman may have a son, but I wasn't clear on that. The rest of the dream is about teaching my flat mate to sort laundry properly and us going to shop for groceries, particularly ice cream. And also something about the bath being really dirty, the tub filled with a kind of black mud. I go to take a bath, and I'm looking forward to relaxing after being gone so long, but find the tub filled with this stuff. Any insights?
There are a few interesting elements here. For one, the change in your residence. I've written before about how houses often represent oneself, and here you are returning to your home only to find it changed. It has expanded at the very least, and there are more people in it as well. So the trick is to look at your life and see in what ways it has changed, grown, and what new people are in it. Have you recently gone through some kind of transformation? A "renovation" of some kind? Keep in mind the woman—and you don't say much about her aside from the fact she is there and may have a son—may be some aspect of yourself rather than another person. Your feminine side perhaps. Which would make the son something you have given birth to, such as a creative project.
Yet all this is overshadowed by your attempts to domesticate your flat mate. Is he sloppy in real life? If so, the dream may simply reflect your frustrations at his inability to manage his side of keeping up your shared living space. But if in real life he is responsible then you must have a deeper desire to communicate something to him, show him something. This is reflected in your "teaching" him to sort laundry. What do you really want to "sort out" with your flat mate?
The ice cream is interesting as well. A sweet, a dessert. In dreams it can symbolize the successful completion of something (just as dessert completes a meal). Your buying it but not eating it, so this suggests the preparation, the anticipation of that success in some aspect of your life. Possibly something you and your flat mate will work on or do together, since you shop for the ice cream together.
You mention, too, that you shop for groceries together (though the focus was on the ice cream). Shopping in dreams points to a desire for or lack of something. And yet here, again, with the ice cream being central, you are planning ahead rather than worrying about more immediate needs. Are you putting the cart before the horse? Celebrating before something is finished? In any case, the dream as a whole shows you and your flat mate collaborating in various ways. Are you doing so in waking life? Or is this dream a reflection of a deeper desire to have that kind of relationship with your flat mate? Only you can say for certain; consider all the angles and decide what rings true for you.
Finally, the mud in the tub. Ugh. Just when you feel like you're getting somewhere and making some progress, you discover this mess. Frustration. Are you frustrated with your flat mate in some way? With your domestic situation? Or even with yourself? Baths in dreams are about cleansing oneself, renewing oneself. Here you are looking forward to that but are unable to do it. I'm only assuming that your flat mate is the culprit for the dirty tub; is there some way in which he is preventing you from relaxing or "coming clean" about something? In any case, the muddy tub points to tension that cannot be released. Whoever is to blame is minor; you must clean your tub and then yourself.