Very common question for gardeners that live in areas where dahlias do not freeze. If they have not frozen, most people wait until sprouts appear above the ground(they may not because of slugs and snails) and then decide whether to dig and divide the clumps or just let them grow another year. Each year they are left in the garden without dividing means a larger clump forms that is more difficult to divide. Since you are in new house you may have lots of other chores and leaving them in the ground another year sounds tempting.