Possible residential floor area

I made a mistake regarding the floor area ratio; it is 0.5.

My brother and I are planning a semi-detached house with 3 or 4 housing units to move in together with our parents, and therefore we need as much buildable area as possible.

Interessanter ist bei der Grundflächenzahl (grundsätzlich), wie weit sie ggf. überschritten werden darf, wenn die Einfahrtsflächen etcetera mitgerechnet sind. Im speziellen Fall von effektiv 269 qm Grundfläche und 462 qm Geschossflächen sehe ich auch ohne Erbsenzählerei und Tricks unzweifelhaft ein Haus bequemer Größe möglich, soweit nicht gerade das siebte Kind noch nicht das letzte sein soll.

My brother and I are planning a semi-detached house with 3 and 4 residential units respectively, to move in together with our parents and therefore we need as much building area as possible..

After all, living nicely with your brother and parents is the goal, for which square meters are a somewhat less useful measure than for the land developer / property developer of condominium units.

I misread the floor area ratio; it is 0.5.

A site coverage ratio of 0.35 and a floor area ratio of 0.5 is a classic ratio for a development limited to one full floor.
It is wise to tell as many details as possible in this forum – not only those you have already noticed yourself that you do not fully understand how to "read."

Two full floors are permitted and a ridge height of 9.5m is specified. Per residential unit, 3 apartments (for semi-detached houses each half-house counts) are allowed. The building area is in Hessen.

My brother and I are planning a semi-detached house with 3 or 4 residential units in order to move in with our parents, and therefore we need as much buildable area as possible..

It is important that more than two residential units are allowed to be built. Often they are limited to 2.
Then it must be stipulated in the development plan whether the floor area ratio for ancillary facilities such as access roads and garages/parking spaces may be doubled.
And then the parking spaces per residential unit are important. Everything must match up.
What does the development plan say about the number of floors and so on?

Two full storeys are permitted and the ridge height is specified as 9.5m. Per residential unit, 3 apartments are allowed (for semi-detached houses each half of the house counts). The building area is in Hesse.

Sorry, I didn't read that when I wrote.

Whether the ancillary facilities may be doubled is, to my knowledge, not evident from the development plan. Unfortunately, the plan cannot be uploaded here.

It is important that more than two residential units are allowed to be built. Often they are limited to 2.
Then it must be specified in text in the development plan whether the floor area ratio may be doubled for ancillary facilities such as access roads and garages/parking spaces.
And then the parking spaces per residential unit are important. It all has to fit together.
What does the development plan say about the number of floors and so on?