Ditto. Been there, done that, called the big guys to build our covenant as well. IMS those tend to be ReTe magi more than CrTe ones. A lot of covenants are built in rocky outcrops that are excavated.
For added fun you can have him collecting arcane connections to his own created buildings. Maybe the "I also create an AC to the building" is part of the spell design on purpose. Just in case 
Xavi
My concept for Citadellius is that he is a servant of the Order at large, and no single Tribunal. With Mercere and Hermes Portals, a magus can travel from Tribunal to Tribunal quite efficiently. It is the travel from the Portal to the Covenant site that is the hard part. So yes, new Covenants are formed infrequently, but taken as the Order as a whole, I -think- there would be enough of them to sustain a speciality.
But if someone has already done the hard work and made an Order-wide timeline that includes the known founding dates of Covenants, I will gladly incorporate that data into my theory!
Citadellus would have been a welcome visitor to the isle of man during the last 50 years. 5 covenants have been founded there during this period.
I do not think he can sustain himself only by creating new covenants. However, he can certainly sustain himself doing that and his normal stuff at home. It is a nice secondary income for a master magus. He might get some renown among mundane engineers as well, since his tractatus on siegecraft and vault-building will have a wide circulation for sure. I can easily see him as an innovator in construction techniques. He does not live near chartres, does he?
Many of the Tribunal books have founding dates for covenants within.
Lol, sorry i mixed it up because i got the impression Mario was the OP. 
Always a good idea, you never get enough practise but more is MUCH better.
And with the Major focus like that, he´s going to be able to do lots of various things and be fairly versatile i think.
Hah, neat little twist.
It certainly does yeah.
Without a doubt i think. It wont be just new covenants, make sure he can do repairs of high quality and he will always be sought after since things break and accidents happen all the time, and then of course there´s the part about adding a bridge and maybe other similarily convenient structures to his repertoire that will keep him in business as much as he wants(obviously from your description he will focus on new covenants, but if times are slow he might jump at other deals as well).
So, he might need some kind of specific spell to get to and from more easily. Could teleporting be used based on an AC delivered to him as part of the contract be an option?
With one specialist "servicing" his own and 1-4 neighbouring tribunals, he would probably have actual work once every few years at least, he wont be making JUST brand new covenants after all, and being so useful in his own covenant, he will likely use his specialty as his service there as well which means he wont actually spend much time on that.
With a bit of hermetically assisted travel when "going to work", this would probably be quite a nice position to hold as it is likely to give a steady income while taking very little time from his own projects or interests.
I wouldn´t be surprised if the order could have a total of maybe 2-4 of this sort of specialists active at any time, probably with a lot more that COULD act as them simply because the Arts needed are very useful overall.
Oooooooooh!! He can be the DEVIL that constructs bridges in one (ritual) night!!! Only for a pittance: unlimited access to your favorite groove of trees (vis source) or that happy cascade in the woods with the nymph.
And then legends start to circulate....
Xavi
How about just parts of buildings that are magical?
Like a self cooking kitchen.
Amusingly enough I've just made a PC magus with a very similar focus, and all without being aware of this thread's existence (minor focus: stone buildings).
As I see it, the work that such a magus could get with covenant construction is likely to be a sideline. Someone with that kind of focus is going to be able to enchant buildings as well. A large lab, spells that allow buildings to be moved, and away you go.
Throw in some sun-duration or moon-duration 'create small stone building' spells, and you can get quite flexible...
Whatever your chosen focus, if it includes the creation of stone structures you also have military options, if you want to put your Hermertic skills up for hire.
Especially if you have Flawless Magic, and a way to teleport long distances e.g. with Hermes Portal.
You could go to some of the Tribunals to the east, and sell instant fortifications to the highest bidder.
A Simple Method for Rapid Vallation, p141 True Lineages (Tremere section) is a good example of this.
Thus you could make money both in times of peace and rebuilding; as well as times of war and destruction.
The rapid vallation spell is one that I would expect most covenants that pass as a mundane manor or equivalent "we do not want to attract much attention regularly" to have in a casting tablet just in case they find some pesky mundane army nearby.
I curiously spend a lot of time trying to build this character concept at Gauntlet. The problem with being a Creo Terram ritualist is that then you need the Vis. Unless you want to give him Imbued With the Spirit of Terram, so he can act as a Terram-vis-battery for his own spells.
In my current incarnation, I have him focused on Rego methods instead, allowing him to use the local building materials to his purposes instead of conjuring magically perfect buildings (I used to have a roommate who was really into Frank Lloyd Wright). Rego Terram as a focus also coincidentally allows for repair work, as you tell a Group of Rubble to become a wall again, etc, etc.
In fact, a Rego Terram, (Touch, Concentration, Group) craft magic spell is a pretty low-level and multi-functional way to achieve the same thing. The time it takes to do the work becomes much longer, but at the same time, the single magus can spread out to similar spells for Herbam and Animal, all aided by his high Finesse, and no longer needs to be an Archmage -- unless you want the Hermes Portal.
However, it seemed like the original poster was looking for this idea as a story element rather than for the mechanical functionality. In which case, we would start looking at the differences between the sort of stories you need to jeopardize a young-ish rising star Master magus, versus an Archmagus (and let's not forget the warping he's accrued along the way!).
I think the value of the story here is in the flaws. What would his weaknesses be? What sort of orthagonal direction can you attack him from?