what is the question tag of the sentence: She had a flat in Paris?

A positive statement of fact: "She had a flat in Paris."

The emphatic (positive) form of the same statement: "She did have a flat in Paris."

The negative form of the same statement: "She didn't have a flat in Paris."

A simple, untagged question that doesn't expect any particular answer: "Did she have a flat in Paris?"

A question using a tag and expecting the answer "yes": "She did have a flat in Paris, didn't she?" or "She had a flat in Paris, didn't she?"

A question using a tag and expecting the answer "no": "She didn't have a flat in Paris, did she?"

To form a question based on a simple statement, we often first look at the emphatic form of that statement. This is because we want some form of the statement that has more than one word in the verb. If the simple statement already has more than one word in the verb, then we don't need to look at the emphatic form. The two-word verb in your example would be the emphatic "did have".

To make a question that doesn't use a tag, simply move the first word of the verb to the beginning of the sentence.

To make a question that does use a tag, first make the statement that you expect is true, then ask the opposite question at the end. The subject in the tag should be a simple pronoun, and the verb in the tag only needs to include the first word of the verb in the main part of the sentence.