These place words are used as مُضَافٌ: another noun completes their meaning.
فَوْقَ الْبَيْتِ — above the house
- فَوْقَ is the adverb of place and the mudaf. It has no tanwin.
- الْبَيْتِ is the mudaf ilayh and is majrur.
Do not read فَوْقَ alone and leave the listener asking: above what? The following noun completes the relation.