# Do we need to use an ancillary qubit when decomposing arbitrary $U(2^n)$ gates using Clifford+T universal gate sets?

As I know, we can decompose $$U$$ without ancilla if it's from special unitary group $$SU(2^n)$$. Do we need to use ancilla qubit on decomposing arbitrary $$n$$-qubit $$U$$ using Clifford+T universal gates set?