The place where we want to recite our Salat, must fulfill the following conditions:
1. The place must be Mubah, meaning that it is not stolen property.
2. The place must not have any movement; for example, we can not pray our Salat in a car or on a train while it is moving, except in the case of necessity (such as if the entire time of the Salat we are on a moving object).
3. The place must not have a small roof such that we can not stand up properly, or that we can not do our Ruku or Sujud the way it is supposed to be performed.
4. If the place where we want to pray is Najis, it must not be so moist that the najasat gets transferred to our body or clothing.
5. The place where we put our forehead must not be higher or lower than four closed fingers, in relation to where our knees are.
source : http://www.maaref-foundation.com