Improvised shelter is certainly worse than specialised one (firm, well-built house). Still, any shelter entry in the table "Equipment Modifier" probably implies some efforts from it's occupants' side. Temperature inside any of the best houses will eventually become equal to that outside without some source of heat1.
But if fireplace is supported functional, I believe it is safe to assume level 4 protection from cold. Characters may stay inside without any additional protection in most2 unerthly cold climates indefinitely (or while Secure Shelter spell lasts).
Without functional fireplace most a shelter in cold climates should be capable of is negating Wind Chill, Whiteout and other hazards related to blizzard, sleet, hail, snow, wind, etc.
Similar reasoning may be made for hot climates. Yet it is only digging into the sand or erecting a tent, and it is already improvised shelter. Level 4 protection from heat seems quite reasonable, maybe level 3 with a lit fireplace.
Also, proper stone house should provide reasonable protection against sand, wind, and sun dangers.
1 I assume the house is too big to be heated by characters' boby heat.
2 There should probably be a band analogous to burning heat for cold enveronments somewhere, as "-50o
F or less" includes everything up to absolute zero. My statement do not take such climates in consideration.