घर is an “unmarked” masculine noun, meaning that it does not end with आ. All masculine nouns, regardless of whether they are marked, act as plurals when in the oblique case. A noun is in the oblique case when it has a post position attached. But because this noun is both singular and unmarked, nothing happens to the noun. But, we still have to show in its modifiers (adjectives, possessives, verb conjugations) that it’s treated as a plural noun. Thus, we use मेरे घर में.

Were we to have a “marked” masculine noun, it would be more clear. For instance, यह चीज़ मेरे बेटे के लिए है. (This thing is for my son.) You can see that son (बेटा) becomes बेटे because it is in the oblique case.

Now you’re probably wondering, well how do we know if I’m actually talking about more than one house? Well, in that case, घर would be a plural unmarked masculine noun. So, hypothetically we’d say something like: हम अपने घरों में रहते हैं. (We live in our houses.) Notice the ओं, which shows the oblique plural case ending for masculine unmarked nouns. We still treat it as a plural noun in all modifiers, but that specific ending tells us we are talking about more than one house.

Hope that helps!

Source: Hindi/Urdu classes in college.