I was hired 3 years ago by a local painting contractor. Recently my
employer started remodeling his own house and this is where the work
is taking place.
This is interesting. If he is doing the work at his own house ...
siding, roofing, framing, building decks, drywall, interior trim
including cabinetry and more.
The building of decks is something that local governments watch carefully. If it is done wrong then people can die. Of course if the work you were being asked to do is gas, electrical, and water then the building inspector will want to make sure everything is up to code.
Even if you aren't doing the electrical the boss could be having a carpet layer do that.
Is this legal?
Maybe. People do DIY projects in their own homes all the time. The work has to be done by licensed insured people for only a small subset of a big project.
I also assume that if the work requires building permits and inspections then those have been done.
Now if you are asking can you be forced to do this....
Can I say no? I was threatened with being fired for not wanting to
perform these duties.
That depends on the contract you have. There are examples when the "boss" requires employees or sub-contractors to do a little favor at no cost to keep the work flowing to the employee. But if you aren't getting paid that is a problem.
Also if your boss isn't the owner, then owner might not know this work is being done and your pay is being miss-charged to another project.
But if the work is legitimate and the boss is paying for the work, and you are getting paid the proper amount then most people don't see a problem.
If you have a contract and can pick and choose which projects you might be able to skip this one. But you still put that contract at risk.
I guess you as an employee could say no. But if there is no other projects you can switch to, then you might see a smaller paycheck or find your self unemployed.