Just want a general itemized list of potential costs to look into, such as cost to develop land, build foundation, permits, delivery and building of modular, hooking up utilities, etc.Thanks in advance!

Financial Transparency:Managing the budget yourself allows for greater transparency in costs, helping you to identify where money is being spent and find opportunities for savings.It's important to consider the potential downsides, such as the significant time commitment, the need for substantial construction knowledge and experience, the risk of costly mistakes, and the responsibility for managing permits, inspections, and compliance with building codes.

The city makes it hard and you have to go in front of the zoning board for approval...but it is possible.Alot of rules and permits to buy- but it would be worth the hassle.

Based on my calculations and preliminary research, I have found that the Bay Area offers good returns from developing land, although acquiring level land and obtaining permits can be challenging.I have a few questions:1) Is it recommended to purchase a complete house plan from online sources like ePlans or similar websites and then customize the architectural design before submitting it to the municipality?

Building an ADU is hard, difficult to get financing and city will hold you up with permits.

The only permit requirement is for sewage treatment.  

No square footage was added, so I didn't pull any permits.

Both FNMA and FHLMC permit this, under the following circumstances: Notes: For all such transfers affecting mortgage loans purchased or securitized by Fannie Mae on or after June 1, 2016, the transferee is not required to occupy the propertya limited liability company (LLC), provided thatthe mortgage loan was purchased or securitized by Fannie Mae on or after June 1, 2016, andthe LLC is controlled by the original borrower or the original borrower owns a majority interest in the LLC, and if the transfer results in a permitted change of occupancy type to an investment property, such change does not violate the security instrument (for example, the 12 month occupancy requirement for a principal residence).The servicer must notify the borrower that a property transferred to an LLC must be transferred back to a natural person prior to any subsequent refinance application in order to meet Fannie Mae’s Selling Guide underwriting requirements.Citation: D1-4.1-02: Allowable Exemptions Due to the Type of Transfer (04/13/2022)Now, you will still have the Note and reporting in your name individually and will still be personally responsible for the mortgage and most likely, the manner of transfer will enable any litigant to pierce the veil of your LLC, but you'll have what you wanted, for whatever reason. 

Engineer and architect to verify buildable and permits able to be pulled 

In most places you can only do work requiring a building permit if it is your primary residence or unless you are a licensed and insured contractor.