What is the lowest BMI at which it is still recommended to have a BBL?

No minimum BMI exists.  No two of us are exactly alike - have same body type and frame. It is a rare individual in whom I cannot find enough fat to perform a BBL.  I have liposuctioned models and thin patients with good results.  Of course, I would need to see photos or examine you to determine if you fall into this category. 
Kenneth Hughes, MD, ABPS Board Certified Plastic Surgeon
Los Angeles, CA

Hello dear!!
There is no minimum BMI to performe plastic surgery. I will recommend you to post pictures to give you the best advice, based on your figure and your goals; also an evaluation is mandatory to give you recommendations.
Good luck :)

Typically, thinner clients are ideal candidates for a high definition liposuction BBL.If you are a candidate for a Brazilian Butt Lift (BBL), the optimal outcomes are achieved with: 1) use of VASER liposuction to harvest the fat, 2) a closed looped system such as Puregraft to process the fat, and 3) strategic micro-packet transplant technique. These three processes have demonstrated over 95% fat graft take. This means that your results will be stable as long as you maintain your weight. As an official VASER trainer, I am routinely referred unhappy BBL patients. One of the most common reasons for disgruntled BBL patients is that patients are not able to accurately communicate their desired buttocks goals to their surgeons.  A buttocks assessment tool can be an invaluable tool to communicate to your surgeon the exact buttocks size and shape you are desiring. A Skype or Facetime consultation with a BBL specialist can help you get an accurate recommendation and pricing. 
Respectfully,
Arian Mowlavi MD,FACS
Realself 100 Hall of Fame Inductee