I am trying to figure out a cost-effective way to add a new foundation to an old (1960s) block and base foundation. This is quite an old style of foundation and in case you are not familiar it's like this, except the one I'm looking at has tree trunk (blocks) instead of cement blocks as shown below:

The layout I'm working on looks something like this and I'm 

I've spoken with a number of structural engineers and gotten different answers from each one. Here are the options:

1. Some say that you need to use the same style piers on both the old and new sides. 
2. One engineer says the same style piers on both sides need to go down 12' and some say that 12' is just too much.  (12' deep piers on both sides to me seems like the safest way but it's also unnecessary over-engineering and extremely expensive). 
   
3. One engineer says that all you need to do is level the old foundation and pour a slab on the new side. The two foundations can be leveled and joined with rebars. (this is the most cost-effective solution). Being in Texas, gound movements happen all the time and good gutter system and a foundation warranty should be enough. 
   

Has anyone had any experience with adding square footage by combining 2 different foundation styles? 

Curious what are the minimum requirements for a city typically just so I understand the options better. 

Thanks!