The basic rule for planting bulbs is to plant them at a depth equal to three times the height of the bulb and to space them at three times their diameter. In very cold climates (zone 3 or less), it may be helpful to bury hardy bulbs deeper than normal, up to a depth equal to five times the height of the bulb. That way the bulb will be better protected against the cold.
It is also worthwhile making am exception for hybrid tulips, like Triumphs, Parrots, and Darwin Hybrids. You’ll find that they will be more more perennial and better protected from squirrels if you plant 1 foot (30 cm) deep. That’s about twice the usual recommendation.
Even so, only plant tulip bulbs extra deep when you can offer them loose, well-drained soils. In heavy clay soil, which the tulip’s spring shoot has more difficulty piercing, it is better to stick to the original rule of three!