If you are using an exterior seedhouse and it’s cold, build a small second
covering on one of the tables, then place a small electric heater beneath it. That way, rather than trying to heat a large greenhouse in cold weather, you’re only heating the small area for a few flats of new seedlings.
A seed flat can hold 400-800 new seedlings, and for that first critical 2-3 weeks during germination, emergence, and formation of their first true leaves they must have uniform warm temperatures – but they take up very little space. They also don’t need light until they emerge from the soil, but then they need maximum light IMMEDIATELY.
Do your first transplanting when they have their 2nd set of true leaves. Then you will need 10 times the number of flats you started with. It’s usually a bit warmer by then, plus the seedlings can handle cooler weather than germinating seedlings, so with a little heat at nights you can usually make it.
If you are starting seedlings in your house, unless it’s being done in a room with temperatures in the 70’s, the soil may be too cold for good germination. An electric germination mat will keep flats warm, and costs $30 for a single flat size and $70 for a 4-flat size at https://www.greenhouses-etc.net/ghse-sunshine/heatmat.htm. You usually only have to use it until the plants are established – then they can put up with somewhat cooler roots.
But remember that plants need at least 60 degrees fahrenheit in order to grow, and most of them go dormant at 50 degrees – especially the warm weather ones – so maintaining warm temperatures continues to be very important, especially in the daytime.