LMSC In the sense of ‘house’? I live in an old house (built in various periods, but most of it between 1690 and 1820). It has traditional uninsulated walls built of brick and stone, plastered with lime plaster (and in some places battens and plastered laths on top of the brick, forming a sort of old-style cavity wall).
The standard value for thermal transmissibility for “solid brick” walls (the closest available option in most software) is taken to be 2.1 W/(m2 · K); actual measurement in-situ of old walls with lime plaster gives values of U of about 1.4.
Standard thickness for ‘solid brick’ wall is 220 mm - 2 bricks - which was a standard construction for terraced Victorian cottages. My house has walls that taper significantly from ~550 mm on the ground floor to ~250 on the top floor.
The end result is a significant underestimate of the house’s energetic efficiency. See for example http://www.wtbf.co.uk/newsite/_blog/app/web/upload/Wall%20Insulation%20Conference/3%20Caroline%20Rye%20U-values%20%26%20Retrofitting%20Traditionally%20Built%20Walls.pdf (from which I took the “1.4” value above)
Similar story for windows - we have large, single-glazed windows. Depending on how sophisticated the efficiency calculation software is (e.g. “freely available” vs. paid-up version), it may make assumptions on the surface area of the windows vs. the rest of the exterior… which in our case overestimates the efficiency, as the windows are particularly large. However they are also well draught-proofed, resulting in a U-value which is significantly higher than that of an ‘average’ single-glazed window. https://www.buildingconservation.com/articles/thermal/thermal.htm
And so on…