For other variables such as tax rates and heating costs, I find it easier to get a good evaluation.

Home was built in 1900, so no central heat and air.

Have the info on:    - any claims in the past 5 years    - when were the Roof, Electrical, Plumbing, & heat last updated    - what alarms/security    - who the prior coverage was for the past 5 yearsCA has become a difficult market for coverage lately so I would start the process early to make sure you are quoted in enough markets.

This would include about 1,000sqft of basic office and restroom and the warehouse area being semi heated only.

This is the simplest method, but it also means your tenants are more likely to abuse the utilities by leaving windows open with the heat or A/C running, leaving lights on, ignoring the toilet that constantly flushes on its own, etc.3.

The tenant never told me the gas was shut off and the heat was not on.

How is the heating system, the a/c? 

I am sure that this house will appreciate really nicely over time.Hopefully the spring market will heat up in your area and that will also help provide some excellent sales comps to boost your value. 

    - fire or burglar alarms    - underground tanks (active or inactive)    - knob & tube or aluminum wiring     - fuses5. what are the years that the Roof, Electrical, Heating, & Plumbing were last updated6.

The formula I would use is 5 total occupants small house pays 1/5 large house pays 4/5.if the heat is also gas I'd do it by square footage - 3/4 large home 1/4 small home.The formula is only part of it.