If it's just an "old building" without any architectural significance, history etc then yeah I'm all for replacing it with something more beneficial as long as it's congruent with the character of the surrounding area.

All one story, will be average quality construction, and is in a section of the municipal City of Los Angeles that does not have architectural committees or anything else outside of normal building codes.Assuming no major issues with adding to the existing dwelling, trying to get a general idea of per foot cost.

It is a box of a building, with no thought whatsoever put into the architecture, no windows, and you walk into one huge room of slot machines.

Any design or architecture student could do this for you really easily and for way less than staging.

As I am an architectural consultant. 

I actually have a lot, worn architectural plans for sale in an up and coming area on the island. 

I say for any structure plan about $125-175/sqft this exclude well, septic, foundation that is additional cost and each one of those can range between 10-30k dependent on soil type depth need to drill and the type of foundation needed to support the structure.I have an Architecture and engineering background feel free to touch base anytime you want. 

I think you are looking for "architectural" support. 

A large part of the value is what you can build with that lot (zoning laws), location, architecture, cost of the build (local builder), and comparable sales or rent in the area are for that structure.Example: Land = 50k, Build = 125k, and if comparable sales are 275k then you could net some profit.Not perfect math but some rough estimates can give you a ballpark estimate if it is or is not worth it.

.#3 Contact Local Contractors for Labor PricingCall local Subcontractors and get budget pricing for common repairs on your typical rehab project.For example, call a roofer and ask what their average cost per Square of Architectural Asphalt Shingle Roofing would be on a 1,500 sf house, with a 6/12 pitch.Note: Some contractors will be reluctant to share pricing without seeing the property, but tell them you are just looking for a rough budget number you can use on future projects.#4 Compile Your Prices into a Spreadsheet or Estimating SoftwareOnce you start to get a better understanding of Labor and Material costs you will want to store this data into a spreadsheet or software that you can use to help you streamline the estimating process.The Book on Estimating Rehab Costs comes with a spreadsheet that you can use to help you populate your own database of labor pricing and material pricing.