Craft magic rules are in HoH:S, pg. 60.
Watching someone actually build this sort of thing in his backyard (Not in terms of size, but basic technology) - https://www.youtube.com/watch?v=P73REgj-3UE&list=PLGnWLXjIDnpBR4xqf3FO-xFFwE-ucq4Fj&index=2
So, that's my plan: have St. Avery build a square roman villa-style structure; have a dry pit around the outside (to source most of the dirt from), with a pretty thick outer and main walls; probably 10 feet high or so, and 3 feet thick laid out in a square, made from adobe/mudbrick. The north side has a large meeting hall, flanked on each side by rooms for the magi. The west side has the servant rooms, the kitchen, as well as a stable. the east side is for storage and additional miscellaneous rooms. The South side is the entryway. The roof is made using the wood from the huts, with adobe tiling - not strong enough to walk on, but can keep out the rain just fine.
If he runs out of wood (which will be the main issue), he can have one of the other magi creo up a bit - for interior separation walls, doors, or (pure wooden) roofs over the servant and/or storage quarters. (ie, make sure there isn't any natural adobe tiles sitting on top of magically-created wood.)
Note that a base 3 effect can summon a tree for conc duration. (So a lvl 5 ReVi Maintain the Demanding spell can maintain it for a month.) So we may have to summon a bunch of tall, skinny trees and rough-hew them to create beams for roofs and whatnot; doable, but may take a few hours.
The spells in question are essentially just Rego Terram craft magic to create the adobe walls and tile; Rego Herbam to move and pull apart the wood huts for buiding material. The base for Stone is 4, and this is mostly dirt and clay (which would be a 2 or 3); working with processed plant products is ReHe 1, so that should be pretty darned simple. St. AVery can /5 spont any lvl 4 Rego effect, so as long as he's not working with stone he can actually work pretty fast. And making adobe and digging ditches and builing a 1-story structure...really isn't all that hard. (Complete amateurs can do this after a day of training - it just takes time.)
Each room has a fireplace and chimney, as these are actually pretty easy to build with adobe. CrHe firewood is easy enough to come by (Bastion can make it, and St. Avery can maintain it) - furniture is unfortunately limited to clay brick counters (in the kitchen) with open-air storage underneath, unless someone wants to try to /5 spont a lvl 5 "work with unrefined wood" to try and make cabinets. Although in a fit of inspiration, St. AVery will attempt to build heated sleeping surfaces, as shown in the video above. (Heated floors were a thing for the Romans, so it's reasonable that St. Avery could be familiar with the concept.)
In looking at the Craft Magic rules - in terms of Finesse rolls, most of this would probably be trivial (3) or Easy (6) - compared to, say, forging a sword, which I'm assuming is Average (9). Of course, I didn't take any Finesse, so it'll be just his Intelligence(5) - but thankfully, this likely isn't a stressful situation, and adobe (by its very nature) can be recycled back into your next batch if you screw it up.