We can’t give you a straight answer, because this depends on two things – what type of pants you have and how warm you want (need) to be. If you have an insulated pair of snow pants, you often won’t have to wear anything underneath.
The insulation itself usually gets the job done on its own, and you would be too warm if you wore a base layer. Additionally, pants are often lined with fleece which feels soft and comfy on the skin. However, sometimes the pants have subpar insulation or the weather outside is so cold that you want to wear one more layer anyway.
On the other hand, if you have shell pants you will usually have to wear an additional layer. Shell pants will give you fantastic protection from water (snow) and wind, but won’t do much good when talking about warmth. This is why you need to invest in a good base layer to compensate for the missing insulation and retain some of your body heat.
When choosing a layer that you’ll wear under your pants, avoid materials like cotton, which is terrible when wet. Instead, you should go for something made of synthetics or, even better, wool. These feel great on the skin, and don’t irritate it. They efficiently keep the warmth close to your body but are also able to wick away sweat so you stay dry underneath. In addition, they also have a mild compressive effect, which helps with circulation and performance.
If you plan to layer under your pants, make sure you get a regular or loose fit so you have enough room for the additional clothing. Even though layering under the pants is not as important as layering under a jacket (keeps your body core warm), doing it can be useful in many situations.