What kind of lavender do you have? That makes all the difference, depending on how cold hardy that particular lavender might be.

I have Hidcote english lavender, which is hardy down to zone 4 (-20 F) but many french lavenders are not as hardy. Since you're in zone 7, many lavenders might be alright...but I would check, if possible, to see what variety you have. Also, it might help if you don't prune it or cut back the old flowers, if you're not sure. (Cold air can sometimes get into the roots, with recent pruning.)

FataMorgana and others will have better info...but I hope this helps :)

Lavender_lass is correct. Knowing which lavender will make a huge difference in what we have to say. I seem to recall some of the spanish lavender types being hardy to zone 8, which is tender even for you. I don't grow those types since they are way out of zone for me so someone with more experience on those can comment. But regardless of the type, excellent drainage will be the key to winter survival. Any place that water pools in the winter will mean a sure death for these plants.

I have lavender (very hardy english types only) come back for me each year. They survive the cold, ice, and snow of my region so overwintering outdoors, so long as your plants are hardy for your zone is very possible and rather easy.

FataMorgana

Your English and Spanish lavenders should be okay in your zone, but do give them a nice, dry mulch for winter protection. If you have any French lavenders, that may be a problem - they seem to be the most tender to frost and freezing temperatures. Often my French lavenders will overwinter (zone 8a) but I have lost them to the cold on a couple of occasions - I've gotten into the habit of rooting some cuttings in late summer/early autumn; if I lose the mother plant to winter, having some rooted stock to replace it with in the spring seems to take the sting out! If I don't lose them, I've got some lovely gifts to give out...

Ditto what has been said about the exceptional tenderness of French lavender but the other lavenders will be fine. Unless it is a particularly tender variety--leave it outside, lavender HATES being indoors.