A finished Tiny House can have a custom layout that has a kitchenette, a space for a bed, basic furniture, a shower and a toilet. It can be heated with an electrical wall heater, propane stove or wood stove.
I prefer to get most of the cedar from a local salvage mill. All other materials are new from the building supply store. If possible I use used materials but just on non structural parts of the construction. The windows are custom made vinyl double glass.
I recommend two electrical setups, 12v DC circuits for LED lighting and 12v appliances and AC circuits for normal outlets, household appliances and heating.
Another option is to have the electrical set up in a way, that allows to add solar panels and a battery bank later down the road.