What is your budget for changes? 100K 300K? 500K?

Renovations cost money. They do not really add value unless you are adding conditioned legally habitable space. Even then, the cost to do so make it a wash in the end as the best case scenario compar d with investing that 100K.

http://www.remodeling.hw.net/cost-vs-value/2018/

I think you're facing two conflicting goals here:

1. Your house is too small and you want to enjoy it more.

2. You want your house to earn for you.

These goals conflict because if you are adding on for your own enjoyment, return on investment is the enjoyment, not monetary. Whereas if you are renovating to take advantage of rental income, you wouldn't realistically expect to enjoy that space.

Figure out which is the more important of the two priorities, then consult professionals about what kind of budget you need to plan for. Hint: most garages aren't built to bear the weight of a second story $$$$

Tough to say without know your COL area but I would venture $80-100 a sq ft for it to be built properly.

If you want an apartment over the garage, you're going to have to tear down the existing garage and build an entirely new structure. And you already have to many roofs so don't add another section without re-roofing the whole thing. It will looked cobbled together.

A garage rental apartment is a small house with a medium house cost. If zoning even allows it. And that would be tear that thing down and start over. If zoning doesn’t allow it, then you are out of luck entirely.

The most "bang for the buck" would be to use the money that you'd spend on these really really major renovations to just move to a bigger house that suits your needs.

I know nothing about resale stuff or construction... But I do love screened and shaded porches, 3 season rooms, and so on. So I will say yes, go for it- a nice porch can indeed add outdoor value and enjoyment.

To John.. where are you? Is this a porch that sees cold winter, or might otherwise need or want for winter shutters? In the summer when the want of the breezes are on- is the air circulation around your yard breezy enough? Or too breezy and you might want some semi-walls or such?

How much do you use that deck and door already for regular day to day foot traffic? If you enclosed the deck area, where would furniture go for sleeping on?

Is the house paid for? Maybe the budget should be used to relocate and either sell or rent this one out to help with payments.

Then up your budget and hire an architect.

If you don't want to move, and the location and weather works for you. Go for it.
Yep, hire a professional to scope things out. Even if you are one and/or know folks, ect. Often wise to hire on a professional eye early on- if only to double check work/notions/plans.
If you do hire on professionals- licensed, bonded, insured. And develop a positive relationship with your local building inspectors.

From the third photo down it looks like you could have the potential to jump the deck with the vertical rail over to the horizontal fence potentially? If so, that might give you a neat little bed nook where the horizontal fence area is while still keeping regular foot traffic directed more to the current deck to door access.

In the South, a screened porch would be essential to me if I wanted to spend any time outside. Even in New Jersey, where my aunts lived, the only way to be outside in the summer and not be devoured was by sitting in their three season room aka screen porch :--)

It looks as though there is a significantly larger house next to you so I am assuming this is in an area that is in high demand. I am located in Los Angeles where homes located in desirable locations are essentially either torn down or remodeled down to the stud with additions - there are certain benefits in Los Angeles by remodeling within the original footprint.

I do second getting a good architect in to design the best use of the space as well as speaking to a knowledgable realtor regarding what the comps support for your neighborhood.

It seems to me that the best thing to do is research what zoning and code do and don't allow before you start making plans. I'd also talk to an engineer (not an architect) about what is possible, what is likely and what is out of the question. Architects often draft lofty and impractical solutions that are really cool but hard to implement, especially on a budget. Engineers are more down to earth and they can tell you what will work. Find that out and that will winnow your questions down.

http://www.charleston-sc.gov/zoning

https://www.ellsworthhallett.com/ Go see these guys. You won't find better.