Stuart is correct in citing the lack of any hard data and, therefore the inability to talk "numbers".
However, the answers to your questions, conceptually are:
1. The value of site approval is tied to its current zoning and where it goes from there. If the site's current zoning meets the intended use(s) for the development of the planned project (the use by right also mentioned by Stuart), little or no value would be created through the approval process. If the site requires a step or steps to get it entitled to accommodate the planned improvements, each of steps has the potential to increase value.
Annexation, zoning, density, preliminary, and final plat approvals are examples of the steps associated with increasing site value.
How much value is created is determined by deducting the "as-is" value from the entitled value.
Establishing the entitled value involves determining and underwriting the Highest & Best use of the site and applying a residual analysis.
A residual Land Value Analysis is conducted by establishing the built-out value of the project and deducting everything from the total value BUT land.
The generic formula Total Sales or Capitalized Value - a Target Project Proft - commissions & closing costs = Residual Project Budget - Site Cost - Hard Cost - Soft Cost - Finance Interest points and fees = RESIDUAL LAND VALUE
Residual Land Value is the price that can be paid to acquire the property that is supported by the Project planned to be built on it.
One of the most important factors in doing this analysis is an Income-Driven approach. That is, it uses Market Sales and rent rates to establish Total Project Value.
2. Unless and until you have acquired the knowledge, skills, and tools to underwrite the site, there is no apparent way for you to assemble a team and conduct the due diligence and pre-development permitting activities.
If you decide to pursue this opportunity, your first step in figuring out value(s) is to confirm current zoning and uses to see the as-is value. Before you put any significant effort into this, you should reduce your understanding of how to acquire the land in writing with your in-laws.
Once you have site control, you can contact developers, contractors, and realtors to gather data needed to determine usage, and value and seek validation from the permitting authorities about what can be built on the site as-is or with a zone change.
If you do get into this process, the most important issue for you should be to be able to answer your #1 question, which is "How much value would getting a site approval plan add?'
To know that, you'll need to do the residual analysis. I'd be happy to send the pro forma tool along to you if you like to play with uses and the values they create for the land.