<p>Is this a complete sentence? By building houses, carbon emissions can be reduced, in the sea.
I say NO because when you remove the phrase between commas the sentence is not complete. That is, the dependendent clause 'By building houses in the sea' needs to be attached to an independent clause/ main clause.
So it should be instead: By building houses, carbon emissions can be reduced, in the sea, people can live happily. In which case what is left over: By building houses in the sea, people can live happily, is now complete!</p>
<p>Am I right in saying this?
Cheers mate</p>