The phrase "a flat area of" is correct and usable in written English.
It can be used to describe a level or even surface, often in geographical or physical contexts.
Example: "The hikers set up their camp on a flat area of the valley, making it easier to pitch their tents."
Alternatives: "a level section of" or "an even expanse of".
We move on, picking our way across a flat area of rock, the scar that is only revealed at the base of the cliff at low tide.
With what seemed the last of his strength, he boosted himself onto a flat area of wood to keep from becoming ensnared in the amassing wreckage.
A flat area of green serves as the backdrop for a woman's profile.
South of the Srepok River is a flat area of tropical deciduous forests.
York lies in the Vale of York, a flat area of fertile arable land bordered by the Pennines, the North York Moors and the Yorkshire Wolds The city was built at the confluence of the Rivers Ouse and Foss on a terminal moraine left by the last Ice Age.
Osteochondral plugs were excised from a flat area of the cartilage with a metal punch.
The shallow seismic refraction profiling was carried out at 18 sites in Tal El-Amarna, which is a flat area on the eastern bank of the Nile River, 50 km south of El Minia Governorate, middle Egypt.
The scientists also wanted a flat area to minimize secondary effects of the blast, and with little wind to spread radioactive fallout.
After passing several small waterfalls, the stream then (still very narrow at that point) flows through a small patch of primary forest in a flat area upstream of the transect.
It was a flat area, devoid of prepared or entrenched positions.
Dancing Ledge, a large flat area of stone resembling a ballroom floor and managed by the National Trust, is a favourite destination for walkers, picnickers, geologists, and rock climbers – but not many dancers.
