I was planning something like the second option. I suppose the lab will be pretty empty until Osias has spent a couple of seasons to 'install' those or similar virtues.
In this case the virtues come with the construction of the lab due to its design. Similar to "subterranean" - which you definitely can't just add later.
Also worth noting- so far only 2 magi have voted on the new labs being built. The private labs will probably be built by the magi occupying them either way, but the guest labs and vis extraction lab need to be formally voted on.
I was thinking about if there could be plants of virtue (similar to beasts of virtue), i.e. 'better' versions of normal plants and was planning to adventure for seeds of such trees of virtue for Osias lab. A fully occupied garden would conflict with that though.
a garden can always be expanded, and you don't need to grow everything you might find in a laboratory garden.
On the other hand having lots of empty space is detrimental to a lab.
There is also always refinement for installing more things in your lab...
Good. Expanding/changing later is fine.
What would be the natural environment that supports a specialization in Creo (instead of Herbam or Terram, as you propose)?
This will start as a quite dangerous lab, right? Filling the size of +5 with a maximum number of virtues (as you propose) leaves a base safety of -5. Osias will of course go for refinements, but the rate is limited to one per year.
basically lots of plants, especially fast growing ones. If you want to define the garden space you would still need to take the natural environment major virtue and only go for size +3 then use refinement to get space for other virtues... or go smaller if you wish. If you make your lab larger than what you have installed you will need to take the empty space flaw for every 2 levels of empty space (though spacious can e used to accommodate the first level, so you can in essence have 2 unfilled levels and only have to take empty space on the third...
Mmh, let's see. Could Osias start with a 1500 sft sanctum (including space for a garden) that has a standard size lab and then expand the lab to include the garden as well?
Development could be something like this:
-
2 seasons to install a standard lab
Size = 0
Occupied size = 0
Safety = 0 -
2 season for refinement
Size = 0
Occupied size = 0
Safety = 2 -
2 seasons to integrate the garden into the lab (natural environment)
Size = +5
Occupied size = +1
Safety = 1
2x Empty flaw -
2 season greater feature Tree
Size = +5
Occupied size = +4
Safety = -2
Mmh, the targeted size is a problem I still have to learn to consider...
Wow, Ossias is looking to build her Garden of Herisson? 
Most of the labs in the Praesidium have an inherent "Superior Construction" Free Virtue when built, is this no longer the case? That's +1 Safety. Also, "Spacious" minor Virtue (+2 Safety): If building a lab from scratch and if space permits, you may choose to start with this Virtue, without needing an extra season of work. 8) With the above Virtues + 1 Season Refinement, you are all set, no?
Occupied size 5(1), Base Safety is 1-1=0 Set the Safety to be equal to the Refinement. If the occupied Size (see below) is greater than zero, subtract it from the Safety. +1 Superior Construction +2 Spacious, +3 Safety, are you going to Experiment much?
Note that for further Refinement to 2, you have to have used the Lab for a year (basically 1 Year per old Refinement score), correct?
You could also go for Superior Equipment (most Praesidium Magi have something like that, which is a Free Virtue), you could allay your Safety concerns thus:
1st Season Elementary Lab 5 size + Superior Construction+ Spacious + Empty x2; 2nd season, normal lab; 3rd season Refinement, maybe roll for Highly organized and Spotless, install Superior Equipment; 4th season install some minor virtue, Lesser Expansion, say. Final Safety is: Occupied size 5(1), Base Safety is 1-1=0 Set the Safety to be equal to the Refinement. If the occupied Size (see below) is greater than zero, subtract it from the Safety. +1 Superior Construction +2 Spacious + 1 Superior Equipment, your lab ends up with a +4 Safety.
At least I think that's how it works, I might be dreadfully wrong! 
yes, everyone can have, for free, superior construction (+1 safety) spacious (+2 safety, uses 1 size), superior equipment (+1 safety), superior tools (+1 safety), and it would take little time or effort to get magical superior lighting or heating...
Ok, next try.
1st season: Basic lab, size +5, spacious, superior construction, 2x empty
Occupied size: 1
Safety: +2 (0-1+2+1)
Aestetics: -1 (0-1-1+1)
Upkeep: +2 (0+1+1)
2nd season: Normal lab
3rd season: refinement
Occupied size: 0
Safety: +4 (1+2+1)
Aestetics: -1 (0-1-1+1)
Upkeep: +2 (0+1+1)
4th&5th season: install Natural Environment
Occupied size: 3 (4-1)
Safety: +1 (1-3+2+1)
Aestetics: +2 (0-1+1+2)
Upkeep: +1 (0+1)
Is superior equipment also available for the 1st season already?
If I'd take the natural environment already in the base lab - as silveroak initially suggested - I'd have a safety of -1 until after the 3rd season. Would that negatively influence opening the arts of Antonio or teaching a virtue?
It takes (nominally) a day to 'install' superior equipment, which I always interpret as just putting everything in the right place and unpacking the deliveries from the glassblowers and metalcrafters. Whether or not the safety is an issue is up to silveroak, obviously (does it count as 'a season of work in the lab'?) but if he decides it DOES, you can just borrow the teaching laboratory if it isn't otherwise being used.
opening an apprentice is a lab activity. As per apprenticed with the risk of losing supernatural virtues.
Arduino and Aurthor vote YES for this.
Oliverius or possibly Arduino could get you the superior lighting or heating if your arts are not geared for it....for a small fee, of course. 
(9 vis for Oliverius.)
Hi all, Gloriana is lost in Twilight for a season, so Aurthor will have to cast the Aegis in 1144.
Neleos is being gauntleted next year, would a lab for him be available by Autumn?
space wise, yes. He will have to spend 2 seasons installing it, or requesting a grog to do it for him.
Anna will push Fortitude for installing labs.