Since I have a keen interest in Crafting magic, I will point you tothis discussion (shameless self-promotion), where I presented three crafting magi. There is some flaws that I did not fixed in the thread but that can easily be fixed.
Some of the learning points I gather from this previous discussion:
- Check with your troop if Finesse rolls are made with Int or Per and bump this characteristic to +3 (if you have spare virtue, you can try to go to +5, but it is unlikely to happen)
- Although not explicitly spell in the rules, Keen vision does not apply to Finesse rolls. First, it make it more powerful than Puissant (Finesse) since it is a +3 instead of a +2, and second it would have a much broader application (since it will apply not only to Finesse roll but any Awareness and other sight-based non-combat skills
- Magical focus or other virtue allowing to invent or cast high level spells are not really needed: you won't have to Penetrate when you build a house, thus as long as you can cast the spell, you won't have any use for the excess casting total. It might take you one or two more seasons to invent your spells, but you don't need that many and you might trade your service for spells;
- If you want to build really large building, you will have to find a way to build it in several pieces, because the Finesse difficulty is directly linked to the time it would take to build by mundane means the objet/building you are crafting. A castle would take decades to build thus the Finesse roll would be in the 40+ to have it made by a single spell (p93, Hermetic Projects, building a grain barge with a single ReHe(Te) spell has a Finesse difficulty of 30). In the same topic I linked, I made a proposal to have Rego craft spell with a duration of Concentration to have a finer control, thus a lower Finesse roll. It is in the area of agreement by the troop, so discuss that ahead of time to avoid being disappointed.
It is very likely that your mage will have to develop a few mundane skills (like Profession: Architect, or Craft: Mason), so Adept Student might not be a waste of Virtue. In HoH: Societas p60-64, there is a few rules on how to make Masterpieces with Rego magic. For example, having a deep familiarity (appropriate skill of 5) grants a +3 bonus on Finesse roll.
Also, having this kind of mundane expertise could justify how you can use several Rego spell to build step by step a building, with an lower Finesse score instead of a single spell with very high difficulty: your mundane expertise allows you to deconstruct your building project in several small ones, of lower difficulty, thus with a higher chance to product a masterwork. Something that a magus with knowledge of how to build a house would have no idea how to manage.
Just for the sake of example:
Per: +3
Finesse: 7+2 (Puissant) - possible to achieve out of apprenticeship without being completely crippled in other skills
Bonus for familiarity: +3 (Prof: Architect: +5)
Total: +15
If you are willing to spend a season planning your construction, your ST might likely give you an extra +3, for a final +18. P61 of HoH: Societas, ease factor of 18 qualify as daily work of exceptionally skilled artists. If you can fragment your casting so each of them complete the equivalent of a month of work by one man, you will have the construction achieved by an exceptionally skilled artists.
For a magus fresh out of gauntlet, that's quite optimised.
After several years, once Finesse is at 10, you can bypass the season of preparation to achieve the same result. You should have quite some reputation by that time.
You will notice that this package of virtues is relatively light: Affinity + Puissant (Finesse), Puissant (Prof: Architect), Skilled Parens because you will need the extra XP, Free expression because it will help with artistic creation, that is 4 minor virtues, leaving you room to boost a bit your Arts (Terram/Herbam/Rego most likely).
If you are allowed to use slow construction spells requiring Concentration, then obviously Puissant Concentration is highly recommended. Cautious with Finesse as well since you will be rolling a lot of these dices.
You can get idea for your construction spells in Hermetic Projects - The Hermetic Shipyard and in Covenants in the section Magical Alternatives to Covenfolk. Basically, your typical construction spell would be around levels 20 to 30. Not elementary, but they don't require magical focus to be achievable.