It depends on if you consider renovation and extension to be a combined action or two different actions.
Some combinations of things can be taken to be a singular unit. For instance:
Washing and drying clothes is a tedious chore.
Fish and chips is a menu item.
Running and hiding from the bogeyman is recommended.
She applied herself to the task with a little bit of vim and vigour.
If a concept is meant to be taken as a combined unit, then it is treated as if it were in the singular, even though it is comprised of more than one thing.
From Bonnie Mills in "'Milk and cookies': Does the 'and' make it plural?":
First up is this sentence with a singular verb: “His humility and his decency reflects the very best of the American spirit.” Or should it be “reflect,” a plural verb? We need to assess whether “his humility and his decency” are two variations on a theme and therefore one thing, or if these two personal qualities are “different and separable.” They seem like separate ideas to me. A person could be decent but not so humble. Others may disagree with this point of view. An anonymous commenter on the Sentence Sleuth blog, where this sentence was criticized as Criminal Sentence 513, argued, “The author intends them to be considered as a unit, as a representation of a single quality, his ‘goodness.’ I think you can get away with either a singular or plural sense—it’s discretionary.”
There was even more discussion on the blog when it came to the following sentence (labeled Criminal Sentence 519): “Their capture and successful prosecution is what we want”. Some commenters argued that the police had one goal—to put the criminals behind bars—so capture and prosecution represented one idea. They therefore felt the verb should be singular. Others thought these two actions were distinct and the verb should be plural.
I personally feel that the first example is poor, because the syntax of (his humility) and (his decency) is very suggestive of two separate items, whereas his (humility and decency) could be more ambiguous. The second example is better. But, regardless, it's the discussion of intent that's important.
Assuming that no typo or mistake in grammar has been made, we can tell how the author interprets the singularity of plurality of something by the verb they use.
In the case of your sentence, renovation and extension can be seen ambiguously.
For example, I personally tend to see this as something in the singular:
The gutting and rebuilding of houses is a time-consuming process.
The addition of a time-consuming process is also a clue to my thinking process. In this case, I envision gutting and rebuilding as a singular, continuous event that simply has multiple parts.
In that sense, I personally would say:
The renovation and extension of the building was completed in 2017.
There is also a level of regionalism that comes into play here. In North America, most collective groups use the singular, not the plural.
In US English, it's more common to hear:
The team is coming to town.
But in UK English, collective groups frequently use the plural:
The team are coming to town.
So, even though it is a singular team, it is still a plural number of people.
To me and my Canadian-English ears, the actual syntax of your sentence makes the following sound off to me:
The renovation and extension of the building were completed in 2017.
If I assumed that this was not a combined unit, and was meant to be taken as two different things, I would do more than just change the verb.
I can see a couple of possibilities:
Both the renovation and the extension of the building were completed in 2017.
The renovations and extensions of the building were completed in 2017.
In the first sentence, using both and adding another article in front of extension makes it clear that the activities are considered separately.
In the second sentence, turning the singular items into plural items leaves no doubt as to the subjects plurality. (Even if there were only a single extension added, the use of the plural extensions could still be considered conceptually, rather than literally, accurate.)
But it also seems as if your original sentence with were sounds fine to UK-English ears.
So, if you're writing the sentence, you need to first determine if you consider renovation and extension to be a single, combined unit or if you think of it as two different things. If the former, use was; if the latter, use were.
Following that, modify the syntax around it to make it even more clear—if you think it's necessary.