In fact, I have got quotes ranging from $160/sqft to $180/sqft for base structure (bare bone) cost. This is just for 8 feet single story addition, but may go up when you increase ceiling level. Additional architect fee+approval etc. Minimum $200/sqft for structure and essentials+finish material cost appx $100k. Depending on choice of finish materials, it may increase.
For balpark estimate $250/sqft any new addition and $125/sqft existing structure interior remodel.
The property I purchased in 2015 was $900k willow glen. If I spend $350k (max 450k) , I can have 3200 sqft home with 1.25M (max 1.35M) while current market homes are not less than 1.9M level. The issue is that I need to spend carrying cost + one year minimum time to make it work.
Now, confused whether to go for 1500 sqft ( as I already have 1700 sqft home in 10k lot) extension or directly purchase some property with 20% or 25% down payment. Second option is easy, but getting right home is an issue.