I'm considering building on top of my kitchen extension (single story) to
add a new bathroom.

Before I go for planning etc, how do I determine if the existing extension
can support the weight of another floor?

Someone said it's about the depth of the foundations and another said it's
about the strength of the structure being able to support the new weight. I
guess it's a combination of factors so I need to know what the steps are in
making this decision.

Any suggestions on the "process"?