@Andy Chen I'd be glad to help with an overview of the process. @Dan H. reply's above are spot on as far as SB9/SB10, ADUs and issues related to easements etc.
First a question, you say you want to achieve the two buildings on separate parcels; why? What is the specific benefit for you with them being separate parcels? There's generally multiple ways to achieve an outcome, so understanding the why can help with the how.
There are two 'buckets' in development, what you are doing now with getting a 'by right' building permit is ministerial. Ministerial permits are just reviewed and approved once you've met any technical considerations. A subdivision (lot split) is in and of itself ministerial, but it relies on a prior discretionary approval. These discretionary approvals are the second development bucket...when talking to clients I tell them these are things like variances, rezones and master planned communities etc. This also applies to subdivisions in the form of a 'Tentative Map', and unfortunately in the State of CA this applies to subdivisions of less than four parcels....every other State I am aware of it only applies to 5+ parcels. That was essentially what SB9 attempted (poorly) to address; your ability to relatively easily subdivide a 2-4 lot subdivision.
So the process is to first submit for a 'Tentative Map' and get that approved. During this you will address technical issues and guidelines, as well as go to the local community members for their review and input. Their input is advisory only and will not make or break a project, but certain groups will rally to oppose what they deem a bad project...today it isn't uncommon to have both a pro and an opposed contingent of neighbors there turning your project into their personal battle over wanting both more and less density...once you get through this you will end up going to a hearing where your project is voted on...again people can come and weigh in, and if they do not like the decision can even appeal your decision as high as the City Council.
The second part is assuming you are approved, you now need to begin working on your 'ministerial' items...your map, permits to address issues, paying fees for affordable housing etc. If you get your building permits first, then many of these may have already been taken care of, but there will still be some you need to address.
In CA, especially for small projects like this, it is not common to have an attorney involved at all. Your surveyor can do the entire process, possibly with the support of a Civil and Architect if you get into more of the site plan and design components. What is most critical is to hire the consultant team that are local experts on the code. That is really the function the attorney could support is the code review, but if your Civil/Arch have the same experience then the attorney is just doing the same job twice. For what you describe it will likely be required to combine a few code sections to achieve your outcome. City staff will simply tell you what does and doesn't work, not point out additional ways to achieve your goal.
I will say that what you outline will be a more challenging project than some others for a couple reasons, one of which is that you want to create a lot without frontage on a public street. It is do-able, but does come with an additional approval process. Another is because this is MFR and not SFR some of the newer development incentives aren't available, so you won't have them at your disposal. But there are some others that you will have since it is MFR. Overall based on limited info I'd estimate you should be ready to spend around $100K for the entitlement process and probably an additional $50K for the final mapping and permits.
