starting seeds indoors

I've been gardening for lots of years (not about to say how long, it dates me) but I've never done well sowing seeds directly in the ground, except for annual alyssum, hollyhocks (even I can't fail there), dame's rocket, those sort of easy ones. So next year I want to start seeds indoors. I'll use trays, seed starting soil, use a light, all the recommended things. My DH is planning to build a cold frame for me. We live in zone 6, borderline zone 5, we're at 4770 ft elevation so our last hard frost is listed as June 1. My question is: can I move the seedlings outside to the cold frame if I use a heat mat? When can I move them out to the coldframe? Any suggestions would be welcome. Thanks

I think you will love growing all your own seeds, it is infectious to say the least, I am only slightly warmer in winter than you and my spring, warmer weather wont start till about mid May, so I can let you know how I do my seeds from veg to flowers.
For sweat peas, I start them off end of October /November inside, I soak the seeds first in warm water as they have a real hard shell, then plant them into individual pots as they dont really like root disturbance, once they germinate and grow 2 sets of true leaves, I move them to larger pots, as they grow, keep nipping out the growing tips and remove any tendrils that form as this just uses up energy, the pinching out also slows the plants growth as it then spends time sending out new side shoots to make bushier multi stemmed plants, I grow them without bottom heat as being indoors is warm enough for them, around Feb/March, I put them out into the cold-frame, make sure there is enough light getting into the frame, at late afternoon, I throw an old carpet over the glass to keep out the cold frost and remove this each mid-morning to let the sunlight in, I dont open the frame till the temp has risen enough in daytime to ventilate for about an our or so each day as the light gets longer, then gradually allow more daytime light to get in and ventilate more till it is time to plant them out, I open the frame longer and then for about a week, keep the frame open all night to harden the plants off for garden planting out,
Jan/Feb I start to plant all my other seeds as they cant be grown indoors for the same length of time as the peas can, by now I need to add a cane to each pea plant to save them growing tangled together en mass, the other seeds I start are veg, like cabbage, cauliflowers, Brussels sprouts, all the winter greens that need a long season for germination and growing, also start off my summer bedding plants and any other types like Phlox, Delphiniums, and anything else I fancy, by March April all the seeds that are going to germinate have done so and it is time to prick them out into either individual pots, or into seed trays spaced out about an inch inch 1/2 apart, these are all started off with bottom heat like inside my electric propagation, when potted up, I still use bottom heat for a few more days, then cool it down, these go outside into the cold frame by about end April/May when the days are longer, a bit of sunshine and only open the frames enough to allow for ventilation, about an inch on a cool day, half open on a bright day, and fully opened on a warm day, but you need to always offer protection for cold night frosts, end May/June they are ready to go out into the garden, tomatoes etc I have to grow indoors in my greenhouse all year as our seasons are too short to allow the fruits to ripen, last year was a disaster as we never got one single red tomato, once all the seeds/now plants are out in the garden, I then turn the frame into a propagator for any cutting I want to make from established plants, it is just hot enough to help the woody cuttings to form roots, yet cool enough to stop the cuttings wilting, after a few years, you soon learn what you can achieve and when to start planting your stuff, to be honest, my frame is never out of use except when it is emptied for a good clean and disinfection to be done, my greenhouse is the same, but if you will only use your frame/greenhouse for a few months of each year, then it is an expensive toy and I could never justify having it, you can heat them, but in my winters it would be too expensive and I would be cheaper buying my plants. the bottom of my frame is lined with 2 layers of the black woven fabric that you can lay under mulch in the garden, 2 layers help to keep weed seeds from growing through into the warm frame, and also offers protection from frost, I then have a 4/6 inch layer of horticultural sand on top of that, this acts as a drainage layer for watering, it helps insulate from frost and acts as a heater when the temp rises when the seedlings need it through the warmer days, you get frames that have sliding windows, but I prefere to have mine that lift up on a hinge as this gives a better airflow and is easier to get trays/pot in and out when needed, it is also easier to clean the glass window in the top.
You can also insulate the sides if you wish by using polystyrene from old packing boxes etc, try ask some stores to give you large pieces so you can cut it to shape, then hold it in place by stapling it with the same black fabric to the sides to hold it in place, my advice would be to make the largest size of frame you can as your better with more growing area than not enough and you cant knock an extra one up in a rush if you have too many plants to fit the one you have, if you have empty space at first, dont leave the space empty to get cold air in, fill the space with an old bed comforter/pillow or cushion to help fill the cold spot where there are no plants, I never heat my frame as the idea is to keep the plants frost free only, the idea is the seedlings should be large enough to grow in the cooler temp in the frame than from the warmer indoor temp with bottom heat.
You need to learn how to adjust the condensation and heat from the sun/cold on the glass without allowing the plants to go to extreme temps all at the one time, but you will have great fun learning to work it, some frames have glass sides but I dont like this as the plants inside grow leggy all at the one side of the inner frame due to the side that gets more sun for longer, remember even winter sun will warm up the inside quickly due to it beating on the glass, mine has double glazing tops as we managed to get one from the tip (thats where we all take our household rubbish) and we found this window unbroken so we asked if we could take it away, lastly, if you have kids around, the glass top can be dangerous if they climb or sit on it, I am sure your frame will be different from mine and I have just tried to give you pointers as to how it can work for your own gardening skills, the principals are all the same really, it is great fun and will help your garden grow earlier and with a wider variety of plants, good luck. WeeNel.

Thank you for all the great advice! I hadn't thought about using the frame for cuttings. I want to see if I can get rose cuttings to root (only the non patented ones, of course). And I hadn't thought about using polystyrene for insulation, although I had used it for the back of a fish tank that was on an outside wall years ago. What do you think about using a clear/translucent plastic storage tub or underbed storage box for a cold frame? It seems to me that you might not get good ventilation when you remove the top. But if you turn it upside down it might be ok. It has the advantage of being cheap (but probably not for long, considering oil prices) and easy to store when not in use.