The way I understand it you can't build it as a S-LSA. Yours will be an E-LSA based on Vans S-LSA
Go here:
Vans link RV-12
Here's a snip from the above link:
HOW CAN AN RV-12 BE LICENSED?
Van?s Aircraft plans on submitting the RV-12 prototype for Special Light Sport Aircraft (S-LSA) approval. When this is received, RV-12 builders may build and license conforming airplanes in the E-LSA category.
Although the E-LSA category does not permit builders to vary from the kit or plans (no unapproved engines, propellers, etc. are allowed) it does have some very attractive features. In the E-LSA category, there is no ?51%? rule, and no restriction on who may assist in building the airplane. This allows any amount of help, professional assistance or even a fully professional-built airplane.
In February 2008, the FAA stopped reviewing new kits for compliance with Experimental-Amateur Built category. Van?s Aircraft, Inc. is unable to predict the ultimate effect of the moratorium on the E-AB eligibility of RV-12 kits, but from the information currently available, E-AB certification will still be allowed on an individual basis. Responsibility for demonstrating compliance with the ?51%? rule will rest entirely with the builder.