What measures do the magi take in order to locate and hire a stonemason from the mainland?
I'm assuming thhis will be a 2-story house? Meaning that the stonemason will have to have been hired first.
We'll have to send someone, either a companion or a magus, to the mainland, probably for a season. That is, unless my magus brings a stonemason as a companion or grog. 
Yea, that's a good point.
Also, consider that often houses of this era would have a stone foundation with wood on top so that the wood wouldn't rot on contact with the ground. That means: (a) we may need a basic stonemason even for construction of the one-story houses; and (b) there might be a basic stonemason on the island, since other houses on the island may have been built that way.
For the time being, there are no companion-level characters in this saga. 
The walls of the villagers' houses are made mostly of stacked rocks held together by crude mortar. That is the way they are building the small house. They don't use wooden floors for the most part, as it is so expensive on the island, instead relying on dried grass and woolen rugs for isolation. Winters are uncomfortable, to say the least. Some of them have basic stoneworking skills, but not enough to build a safe 2-story building.
We talked about a 2-storey house in the beginning and that is why we need a stonemason. The lower part of the house needs to be built in stone to be able to support a second floor. I don't know if we have discussed how many rooms there should be.
A council room which could double as library and reading room perhaps.
Sanctums for at least three magi, including space for labs. We have a fourth magus coming in but that doesn't the characters know yet. However they may hope for additions and therefore make room for him in the original plans.
Grogs and servants could live in the two smaller houses.
Perhaps this discussion should be taken elsewhere. Arthur feel free to move it where you want to have it.
Moved the discussion (and Bearlord's post) here as it is a more appropriate place for it.
As per the agreement with the Oberland villagers, you can build 2 large houses (2 stories high with some cellars below them). By the villagers' definition, a large house is about 25 by 40 feet. That's about 1000 square feet per floor.
I think the goal with buying the former inn was that it would give more space for the grogs' living quaters, so they only a few of them would have to actually live on Oberland. In any case, the agreement with the villagers was also that half of those working on Oberland would be hired from the village. That might be for kitchen staff, laundry, building maintenance, cleaning, etc.
I'm sure the stonemason will want to have at least basic plans drawn for him when he arrives. 
I had forgotten that we are allowed to build 2 large houses. So it's four houses in all, 2 large and 2 small?
And yes Lothar would like some basic plans, otherwise he will build the houses his way... 
Prochorus takes a stick and sketches some on the ground.
Like that. Nice and sheltered from the wind, sheep are kept away, all the good stuff.
The question that Lothar is likely to have is, what is the interior layout of the houses?
(BTW, who is Arktos?)
My other character. Changed.
How much space is needed for a sanctum and laboratory to be considered decent-sized?
With 4000 square feet + the 2 smaller houses I think we have quite a lot of room even for some of the grogs.
Here is a very rough suggestion for room allocation:
Big house 1:
Council Hall
Library
Sanctum and lab for one magi
Big House 2:
Sanctum and library for three magi
Small house 1:
Kitchen and living quarters for 1-2 grogs.
Small house 2:
Living quarters for 4-5 grogs
A standard lab is 500 square feet. So you can fit 2 labs on one floor of the large houses.
That is separate from the living quarters of the magi, but how much space that would require is really up to you. It can range from just a small bedroom (about 80 sq.feet) to a full-blown suite with a large bedroom with a personal studio or meeting room (250 sq.feet or more).
Just for additional reference, a modern meeting room for 10-12 people is about 200 square feet.
Note that for the villagers, one of the smaller houses is enough for a whole family of 4 to 10 people (often with some livestock). Medieval living standards are much more crowded than what we are used to. One of the reasons is that it preserves warmth in the winter. And in the summer, most of them are working outside anyway. Of course, for peasants a house is a place to eat and sleep, not work.
EDIT: A lab can be smaller than standard without too much impact. If we use the rules from Covenants, it just means it will have a minor flaw, which can be eliminated by spending an additional season refining the lab.
EDIT 2: If the kitchen is away from where the magi will be eating, expect some lukewarm meals in the winter and grumbling kitchen staff.
EDIT 3: The small houses are about 500 square feet in size, giving you a total of 5000 square feet to play with. Plus whatever space you dig up as cellars. Note that diffing up cellars after the building has been erected is riskier, as it can undermine and weaken the building's structure.
Here are some very simple drawings of the floor plans of the two larger houses. As a suggestion.
The labs are only accessible by stairs from respective sanctum.
I have not included the cellars but I think the stonemason should excavate those first and then use the stone as parts of the construction material for the walls.
Ground floor House 1:
Upper floor House 1:
Ground floor House 2:
Upper floor House 2:
That works for me. Thanks for drawing it out.
Dietlinde mentions that having that many people eating in the kitchen won't be practical. Perhaps the room labelled "Guest room" in the second house might be better if it was a hall. Perhaps even merged with the hallway, or simply seperated by an archway of some sort.
How would the hearth be arranged? You were granted the rights for six hearths for your community, as per the document Dietlinde obtained for you. The small houses each require one each, so that leaves only 4 hearths available for the large houses.
And a lab / sanctum without a hearth will be quite cold in winter. And we have to have a hearth in the kitchen, I presume?
If we have one chimney, with two fireplaces (say in the sanctum and the laboratory) does that count as one hearth or two?
There shall definately be hearths but I didn't include so much detail. Two chimneys per large house then?
My thought with the kitchen/dining room was that it may be divided by a wall. I made the ground floors look the same in both houses and just changed the text.
The sanctums are about 200 square feet each, the other bigger rooms might be 250-275 something. The entrance could be directly into one of the bigger rooms and then the hallway could be diminished to a smaller area in the middle.
If we can get away with two chimneys, but multiple locations that get heated, then that is fine. I don't know how the "hearths" limit was understood in that place and time.
Hearth ≠ chimney. In most peasants' houses there is no chimney, just a hole in the roof.
Every fireplace is likely to be counted as a hearth by the tax man.
Then it seems what we have to do is set up a hearth in the kitchen, and one in each laboratory, and for now we each plan to use our lab as our living space. Set up the bedrooms so that if we can arrange to heat them we can use them later?