In a side-split on three levels, four including the basement, the heating and cooling unit is almost new (2 years), and the heating/cooling unit was only available at 35% more cubic feet capacity than the house. Hence it should be quite easy for it to handle the volume of air inside.
Both summer and winter I keep the air ciculation turning on perpetually, whether the heating and cooling unit is on or off.
The thermostat is on the main level, set to about 68F/20C in the winter and a little more, 70F/21C, in the summer.
Despite all these factors, each year in May and September I find it necessary to move my desk, in May downstairs and in September back upstairs. Otherwise the temperature is too uncomfortable for an extended sit-at-the-desk session. I found it easiest to just buy a second desk, and move everything else (monitor, monitor arm, laptop, paraphernalia, desk chair).
- Normal or near-normal. Regardless of the roof and wall insulation, the rooms upstairs will have to heat up if the house (detached) is baked throughout the day from the sun. In the summer the temperature outside is regularly 80F/28C and sunny.
- Not normal. The description above indicates that the walls and especially the attic are not sufficiently insulated.
The bedrooms are upstairs, but the extra heat is not an issue, because the temperature outside is anyway cooler at night.
In this question I'm not even seeking a solution yet (there is already a huge leafy tree on the east side and another, even larger, on the west side). I'm first trying to understand what is to be reasonably expected for comfort inside at all levels.
Quantifying (following isherwood's comment):
I'm asking for a round-the-year issue, but mentioning specific temperatures may help. In this late May, and while the temperature outside is a pleasant 65F/19C, I haven't even switched on the A/C unit yet. Despite that, the upper southwest room (used as winter office) reached 78F/25.5C yesterday. The air circulation is on. Is this heat level normal? Could a room that's baked on two sides for the whole afternoon be expected to remain cooler? (Notice that it became much warmer than the outside.)