Additional floor on existing multi-family house

Dear community, I am about to purchase a multi-family house from the 60s, reinforced concrete ceilings. The roof structure will definitely need renovation in the next few years. I want to take the opportunity to create additional living space at the same time. My plan: take down the roof structure and add a recessed floor made of timber frame construction. However, I can hardly find anything about this on the internet. Why is this rarely or almost never done? You only ever find renewal as a pitched roof with dormers, etc. Why? Does a recessed floor cost significantly more? Approval effort? Does anyone have an idea? Many thanks

Apart from my déjà vu with this thread: I suspect that you are thinking a bit naively (among other things due to a change in the building class, or trivialities like chimney cross-sections, downpipes, etc., which would also need to be considered).

A stepped roof differs visually from a gable roof, therefore it is relevant for building permits -> [Bebauungsplan]
Furthermore, it is quite complex with the ceiling of the top floor (construction and insulation)
Last but not least, the structural engineering changes, which the old building often cannot support.

A mansard floor differs visually from a gable roof,

A mansard storey could also have a gable roof.

Not least, the static load shifts, which the old building often cannot bear.

That is why the quaint solution approach in the linked thread.