Can you add bathroom lighting to the GFCI receptacle circuit?

It depends on whether that 20 amp circuit feeds one bathroom or more than one. NEC 2023 Section 210.11(C)(3) requires at least one 20 amp branch circuit to supply the bathroom receptacle outlets, and says that circuit shall have no other outlets. The exception to that rule is what answers the question: where the 20 amp circuit supplies a single bathroom, outlets for other equipment within that same bathroom are permitted, provided the requirements of 210.23(B) are met.

The two situations

If the circuit serves only the one bathroom, you may add the vanity light, the ceiling light and the exhaust fan to it. That is the exception working exactly as intended. If the circuit runs to the receptacles in two or three bathrooms, which is a very common way to wire a house, the circuit is not allowed to have any other outlets at all, and the lights must come from a general lighting circuit. Trace the circuit before you tap it, because the answer flips entirely on that one fact.

The 50 percent rule

Section 210.23(B) says that where a circuit supplies both fixed in place utilization equipment and other loads such as receptacles or lighting, the fixed in place equipment may not exceed 50 percent of the branch circuit rating. Ten amps on a 20 amp circuit. A vanity light bar with LED lamps and a 60 watt exhaust fan are nowhere near that, so the rule rarely blocks anything in a bathroom, but a 1,500 watt bathroom heater absolutely will. A fan heater unit needs its own circuit.

Wire the light from the line side

Section 210.8(A)(1) requires GFCI protection for the receptacles, not for the lighting. If you feed the light from the load side of the GFCI device, a hair dryer that trips the GFCI also drops the lights and leaves someone standing in a dark, wet room. Land the light and fan on the line side of the GFCI receptacle, or feed the receptacle with a GFCI breaker and pick up the light ahead of it. Doing that also keeps a lamp inrush from nuisance tripping the device.

Switching and AFCI

Section 210.70(A)(1) requires at least one wall switch controlled lighting outlet in the bathroom, and it must be switched at the point of entry. Section 210.12(A) requires AFCI protection for 120 volt 15 and 20 amp branch circuits supplying outlets or devices in dwelling unit bathrooms in the 2023 code, so the added circuit needs both protections. A dual function AFCI/GFCI breaker handles both at the panel and is usually the cheapest way to satisfy the code on a remodel.

Field takeaway

One bathroom on that circuit means yes, you can add the light and fan. Two or more bathrooms on it means no. Either way, feed the lighting from the line side of the GFCI so a trip does not take out the light.