Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance; Flight Crew Landed As Precaution; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport
Aircraft
I was the pilot flying and my First Officer was the Pilot Monitoring.  We departed with normal procedures and call-outs. At 400 feet I; as Pilot Flying; commanded 'Heading Mode.'  At approximately 700 feet I was alerted to a Master Caution light and a LAV SMOKE DET annunciation. I was able to hear the lavatory smoke detector and was then promptly contacted by the flight attendants regarding the alarm.  Passing 1000 feet I called for 'Flaps Up. Climb Power. VNAV.' Once this was accomplished; I stated that I had the aircraft and radios and asked my First Officer to Run the QRH for LAV SMOKE DET annunciator.  I [advised] Approach Control and requested a return to the airfield.  The First officer completed the QRH procedure and the Return Checklist while we were on a left downwind leg for a visual approach. I contacted the Tower and was cleared for landing.At some point on final approach; the LAV SMOKE DET annunciator went out and the audible alarm coming from the lavatory ceased. All crew members in both the cabin and cockpit smelled the smoke.  We confirmed that we were below our landing weight limitation and then I called for the gear to be lowered and the Landing Check accomplished. We landed without incident and taxied clear.We deplaned normally and had the fire department inspect the lavatory specifically and the rest of the aircraft generally.Flight attendants informed me that they saw a haze of smoke during the LAVATORY SMOKE DET. Unfortunately this is my second [situation] with this aircraft. The last time I flew this aircraft I had a slightly dissimilar smoke in the cabin event.My previous event occurred in 2015 and can be found in the aircraft logbook.  Another cabin smoke event occurred in the same month and can also be found in aircraft Logbook.
During climb the crew was alerted to a Master Caution light and a LAV SMOKE DET annunciation. The Crew was able to hear the lavatory smoke detector.