Is a bungalow possible? What speaks against a bungalow?

So my floor area ratio is 0.30, but that just fits with the terrace and driveway, parking spaces, etc., and that's with 900 sqm.

And only 112 sqm bungalow

The question is what exactly is included

That is stated in the development plan

The floor area ratio is 0.35. For plots between 600 and 700, is a bungalow with 140 sqm + terrace and double garage possible here?

Just calculate it!

600 x 0.35 = 210 sqm
Bungalow 140 sqm + wall floor areas = 175 sqm
Difference = 35 sqm
Double garage needs about 36 to 50 sqm
Conclusion: that's not enough even without driveway paving, the terrace (fully counted in the floor area ratio) does not fit in either. From about 900 sqm it would be much more possible.

Since we don't like sloping roofs, we are now leaning towards a bungalow.

I see the plot strongly arguing against that. What does the roof pitch in the development plan say about the feasibility of a one-point-zero-story building?
And: what exactly is the issue with the aversion to sloping roofs: "fear" of only moderately usable areas? (which could possibly be addressed with knee walls).

What speaks against bungalows?
When they get big, they become foolish.
A bungalow for 2 elderly people is okay. If the thing is supposed to get big, you rarely get a smart floor plan. Many long, narrow, dark corridors. Lack of zoning, long walking distances.
And yes, it is generally more expensive than a 1.5-story house (possibly also your alternative, besides two full stories). You save a staircase, but instead you have more foundation/slab/steel/earthwork, a more complex roof. Land area also has a price, please don't forget.
If you want barrier-free access for the day that may never come, plan a straight staircase. A chairlift costs 3-4K €, and then you don't have to necessarily build on one level. Or an option for a partition on the ground floor to be able to sleep there as well, guest WC then with a shower and wide enough.

...Because we don't like sloped ceilings... ?

If I were you, I would interrogate myself about this sentence.
What don't you like about sloped ceilings?
On a scale from 1 to 10...
Do you just want to avoid them?
Would it be nice not to have them?
Is it just a statement born out of laziness because when designing a house _with_ sloped ceilings you actually have to think a bit?

Sloped ceilings also have advantages: the roof resists the wind and prevents a house from looking bulky. A house is only a house if it fits Santa Claus ;)
Besides, the rooms are cozier. With a reasonable knee wall and a good roof pitch, you can combine all the advantages.