The Radak rejects such an explanation, saying: "He saw from upon the
roof that she was bathing in her house." This understanding is
reasonable, both because the roof was already mentioned at the
beginning of the verse, and because if the words "from the roof"
relate to Bat-Sheva's bathing, it should have read "al ha-gag," and
not "mei-al ha-gag." It stands to reason, then, that David saw
Bat-Sheva bathing from atop his house, which in any case was higher
than any other house in the city.
Uriya's house was below David's house, as the verse states: "And David
said to Uriya, ‘Go down to your house,’" and so too several times
later in the chapter. According to the accepted view today, David's
house was at the top of the ridge of the city of David and looked out
over the entire city, which descended southward from the royal house.