Figure 31 of the ADAAG shows the clearances that are required under a lavatory or sink
There are several dimensions that let you know how much space there should be provided so that the wheelchair user can fit under the sink. Notice the 8" min horizontal dimension from the front of the sink back towards the bowl. There is no question that this 8" is a requirement.
I performed an inspection where this 8" was ommitted. The photo below is of the sink in question.
The architect had drawn the correct drawing which showed the 8" horizontal for the knees. Lucky for me, I brought the drawings with me to show the Owner. The contractor argued. He said that this was sufficient for a knee space. Of course this is not acceptable and they will have to fix it (and hopefully the Owner will not have to pay for the fix since it was the contractor's mistake).
Why argue? It is clearly not built per Fig. 31 and there is no question that the 8" was not provided. This profile will not be able to be accessed by a wheelchair. So just fix it!!!!