Do I need to invert the sentence if I move "southeast from the roundabout" to the beginning of the sentence?
Another road is to be built southeast from the roundabout. ✔
Southeast from the roundabout is to be built another road.❓
Southeast from the roundabout another road is to be built.❓
In a declarative sentence in English, the subject needs to come before the verb. So #1 is correct:
Another road is to be built southeast from the roundabout.
But #2 is incorrect:
Southeast from the roundabout is to be built another road (incorrect)
#3 sounds awkward. If the road was already in the process of being built, it would be correct to say:
Southeast from the roundabout, another road is being built.
Otherwise, for the future tense to sound natural, you need a subject and a verb. For example:
Southeast from the roundabout, they will be building another road.