It's a challenger 100 amp panel.. no it's not a Stab-Lok.
There's nothing wrong with Challenger panels. But Challenger breakers were caught in a VW-like cheating scandal; they don't trip when they're supposed to. Only Type C and Type A breakers will fit in a Challenger panel, fortunately Eaton BR breakers are cross-listed Type C, and BR tandems are cross-listed Type A. So all the Challenger breakers need to go and be replaced by Eaton BR/C/A. BR is the most popular breaker type in North America.
what confuses me is I've seen so many panels overloaded when going to customers homes and wondered if I add up all the numbers they exceed the main breaker size how is that?
Oversubscribing panels is allowed because nobody maxes out all their circuits at the same time. Also, USA panels are actually 240V, and so we have two banks of 120V at full amperage.
There's an old joke... someone says to their banker "Overdrawn? How can my account be overdrawn? I still have checks left in my checkbook!" Number of spaces available in your panel is a lot like that. It has nothing to do with the actual electrical load.
Both these issues are resolved by a procedure called a Load Calculation. There is the official, NEC-approved way to do that. Some of the math gets tricky especially with ranges, but the good news is you don't need to tally up lighting or any of your plug-in 120V loads, as those are accounted for in "catch-all" figures.
When you say "Common" do you mean "common" off the timer labeling.... or do you mean "Neutral"? They are not the same thing. Neutral is not "common", common is a hot line actually.
I presume the house was heated previously, so it sounds like you are converting from another heating technology to electric resistive heating. This is a huge electrical load change, and absolutely requires a new Load Calculation. Really any new large appliance requires a new Load Calculation, but this most particularly.
Fully expect to find your electric service is inadequate and that a "heavy-up" would be called for. This would be a great time to address the full panel - a new main panel can be added, and the existing Challenger panel can be re-configured as a subpanel so you don't have to rewire.
Make sure you have a full understanding of your power company's local rate tariffs before installing electric resistance heating as a heat source. This is cheap to install but incredibly expensive to operate, unless you really know what you're doing re: power company tariffs. Resistive heating is cheap to install but prohibitive to operate. Better off with modern heat pump tech.
If your plan is to have the tenant pay the electric bill, then you can't DIY this work - all electrical work on rental units must be done by a pro electrician (who will do the permits, Load Calculation and possible heavy-up as part of the job). HVAC contractors have been known to do "light" electrical work related to their installation, but we see a lot of work that is just terrible.
Should I sub or replace with 150 or possibly 200 amp for future addition to the newly added addition the owner decided to do for an extra apt in the attic to extend the second floor apt.. once I know what to look for in the existing service line.
Nope, you doing electrical work for a third party is out of the question. To DIY electrical work, you must a) own the property, b) be the planned occupant of the unit in which you are working (and NO homeowner is going to install baseboard heat unless they have a favorable tariff), and c) pull permits and get inspections.
It sounds like a) this is for your customer and b) they are a landlord renting to tenants, which explains the cheap-to-build, expensive-to-run electric heating. Of course that particular slice of slumlord bastardry only works if there are individual electric meters to each unit! Which precludes the method you are proposing.
Then my other option is to have a licensed electrician install a new line, weather-head and socket, I can do the rest from there.
It wouldn't be my first time installing a new panel.. I'm very meticulous or anal about safety... something I have to be when it comes to stuff like this..
And yet, you are involved in this dubious scheme.
I've been running a micro business for years with plumbing, heating, electrical and now large kitchen and bath appliances with a very good success rate at not failing stuff I knew nothing about, but with reading here and elsewhere and by the grace of God things turned out well, all homes are intact years later..
Animals collect safety data only by what they witness for themselves. We have very bold pigeons in our local park. Catching one is easy, but if you do, only the pigeons who saw that happen become afraid of people. Whereas human beings are able to share experiences so all humans learn from the experience of others far away from them. So when you say "none of my buildings exploded (that I know of)", that is the animal way of analyzing data. The human way is what the Electrical Code is.
Professionals do real analysis of thousands of accidents. The result of that analysis is the Electrical Code. It's not New York lawyers dreaming up stupid requirements to inconvenience you or cost you money. Every rule is written in blood and ash.