Here is the sample sentence:
We have carried out works for private individuals and property developers as well as our own building projects."
Should there be a comma before as well as in this case?
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.
It isn't clear whether you've carried out works for your own organization (what you refer to as "our own building projects") or if, in addition to the works you carried out for others you've also accomplished building projects of your own (quite different from rendering service to an internal organization).
So one of two things is what you intend to convey:
I suspect you mean the latter, (which is also what I think most people would suspect), but, again, what you actually mean is up for grabs.
If English is a second language for you, please forgive my abrupt style. The problematic language in your sentence is the clause "carried out works". Rewriting that component of the sentence may make all the difference.
May I suggest the following,
To clarify, one of the larger issues with this sentence that people have not discussed is the problematic distribution caused by the preposition "for". "For" creates a prepositional phrase, in this case an adjectival phrase (modifying which 'works'?), as a result, we anticipate that the list would continue to the end of the sentence - including the personal building projects.
Technically, no punctuation is required as others have pointed out. But that does not mean the sentence is a good sentence. The reason it is so difficult to give a firm answer is because the verb doesn't distribute properly, and the punctuation makes it difficult to understand the distribution of meaning. Think of a parallel example: "I have hunted for elk and geese as well as fish." The verb hunt does not work with the act of procuring fish. This is what Kris above points out.
I have searched many sources for the answer to just this question. I do not have to suggest a re-write to solve the problem. I believe the answer to your question is no comma because "as well as" leads to new and necessary information. It does not define the nouns before it or provide the by-the-way information that could be left out, and therefore, require a comma.
EX: I went to the grocery store and purchased eggs, milk, and butter as well as cheese. No comma because the additional noun (cheese) is needed in the sentence to give the full information.
Instead, you would need a comma if it is not necessary information, an explanation, or definition.
EX: I went to the grocery store and purchased meat, cereal, and dairy items, including cheese.