Yes, the house is less than one year old - this is a warranty issue. Get the builder to fix it before the warranty expires. Put the complaint in writing so it's documented - it's bad design and it's going to take some work (and expense) to fix.

Hi Jake!

The plan has always been to get a 11th month home inspection so every issue will be documented and submitted. I'm just wondering specifically why there is such an imbalance. Something that I noticed immediately in the basement was the fact that there was no cold air return vent. It is a high efficiency furnace with sealed combustion if that matters.

"The plan has always been to get a 11th month home inspection so every issue will be documented and submitted."

Bad idea. Do NOT wait until the warranty is about to expire to get this fixed. It sounds like a ductwork/damper issue and these can be difficult and slow to correct (assuming the HVAC sub is the usual lowest bidder).

Get things fixed as they present themselves and don't wait to bundle everything together at the last minute.

You want to fix this as soon as possible. At best, it will be an annoying issue come ever winter or summer when you have to adjust the AC/heat. At worst, it will cost you money. The issue is HVAC related but it might also be that you have drafts coming in from certain rooms. I faced your issue in my own house. The bedroom is nearly 10 degrees cooler than every other room, as it was an addition to the house. It could be because of Poor craftsmanship, lack of proper inspections or drafts etc. In your case. In my case we had drafts due to the house being renovated and a new addition being added which wasn't sealed properly.

At worst, it will cost you money.

Why, though, should it cost me anything in a brand new house?

In most cases when this is the issue, you end up throwing money into heating and cooling the house and keeping the AC/heat on for longer periods in hopes of it fixing the temperature in the house. But if there's a leakage/draft issue, for example cracks In the window sill/door, cold air can leak in and no matter how hot and how long ($$) you keep the heat on, it will still be cooler than other rooms in the house that are completely sealed off. Although rare, new houses (if done shoddily) can still have construction issues. Take a look around windows, baseboards etc see if everything is caulked well. But for your case I would talk to a HVAC technician, as I think it's a duct issue.

Why, though, should it cost me anything in a brand new house?

Your builder will attempt to correct the problem in the least expensive way. The builder may say the temperature variation is within industry norms and there is nothing to do. You then have a choice to spend your own money to add dampers, increase duct sizes, move thermostats, add returns, etc. to achieve the temperature uniformity to your satisfaction.

That makes sense mike_home.

You don't want to wait until the 11th month. Let them know ASAP. They will be all about "fixing" this with as little actual effort as possible.

You want to provide facts and cut through the BS.

Buy several cheap digital thermometers and put them about the same height.

Record the temps, the outside temp, the date and the time and the thermostat set temp.

It could be multiple reasons - bad design, not enough vents, not proper return, not properly insulated, not balanced, etc. We can't figure that out from here.

A home in a cold climate zone like the OP's could see significant floor-to-floor temperature differences as a consequence of stack effect in winter. A single, centralized system without zoning has its limitations.

Balancing dampers may need to be adjusted seasonally if the air flows required to maintain comfortable conditions in air conditioning mode and heating mode are different. I have two balancing dampers in the ductwork serving the 1st floor in my home; they are adjusted to a "summer" or "winter" position to maintain comfortable conditions in the master suite.