How cold can you lager/cold condition?

I have a stairway leading from the outdoors into my basement. The stairwell is covered by steel doors. Right now, when the temps outside are in the single digits, it is 26 degrees in the stairwell. Most of the time, it is close to but above freezing. How cold can kegs/carboys full of beer get without freezing problems?
This would be a natural place for lagering & cold conditioning during the cold months and Vodka would keep the airlock safe.

26 degrees is too cold to ferment a lager. You want to be high 30's-50 degrees usually, depending on the yeast strain.

Cold condition - the closest to freezing without actually freezing it is the best.

Bottle it and put it in a cooler in the stairwell.

The freezing point is a direct result of the ABV. It is indirectly proportional. (higher ABV lower freezing temp)

I did this in my garage and it worked great. Put an oven thermometer or the like in the stairwell, the cooler should keep it a few degrees warmer unless it is below freezing for 5 days straight.

I would not put a carboy full of beer in an area that might freeze. The temperature depression you get with beer is small and the water will freeze out anyway. You can wrap a keg in insulation and keep it to the average temperature. A backpacker's sleeping mat works.

Lagering, you'd be fine, a bit low for cold-conditioning ales, though.