Is money earned from preparing engineering drawings essential for a contracting company in a consulting engineer's project considered lawful (halal) or unlawful (haram), especially if they do not have someone to prepare those drawings?

If your work on these drawings is outside your official working hours at the consulting office, then there is no harm in it. However, if the absence of someone to do these drawings at the contracting company deprives it of the right to execute the work, then you must inform the project owner of that, because you are his consultant.