Learn English – Comma before “by”

In American literature and university courses, the method for introducing a quotation is the same as the British English style: the comma is placed after the last word that introduces the quotation.

Martin said, “Who brought that dog?”

As for the closing quotation mark, American English prefers the comma to be placed before the ending quotation mark.

In her notebook, Sheila wrote that the sun was “beet-red,” but after the game she had seen it “disappear over the bleachers.”

In this context, "as well as" is a coordinate conjunction, combining two clauses into one sentence. A good rule of thumb here is to substitute another coordinate conjunction you're more familiar with, such as "and"; if you were to use "and" in place of this phrase (for instance, "We have carried out works for private individuals and property developers and carried out our own projects") it would not take a comma.

That said, Mustafa is also right; the second clause of the sentence can't stand alone without a verb. Throwing a verb in there as suggested should make everything okay.