It is small - what are your goals for the room? How many guests do you need to seat at a time? Can you elaborate on the built-ins - where they will be placed, how wide/long/deep, etc.

Units will look like the photo attached.
It’s just me and the dog in the house, but I would like to able to sit 5 when friends are over. Sofa under the dust sheet is a 3 seater so would need to fit another 2 seater or two single chairs.

I would probably do four swivel chairs with an ottoman. That would be comfortable for both conversation and for watching the screen, If you have five people, you can bring in a dining chair or a cute pouf or stool.

I think you are good with the sofa where it is. put a slim sofa table behind the sofa for a lamp.

I would do 2 swivel/barrel chairs keep them small facing the FP. get a drop down TV mount

What are the dimensions of your existing sofa? Do you plan on keeping it?

In case you can't spot him.

He's color coordinated!

I would mount the tv over the fireplace and leave the couch where you have it. You could replace the couch with a futon---they are inexpensive but look nice. They are sold at Wal-Mart for about $270. I would get a small coffee table----a floor lamp that you could put in the corner next to the fireplace end. You could get one armchair and sit that next to the sofa. You could get a big artificial tree and set that on the opposite side of the chair. Across the room from the chair.

Here is a possibility.

Do you have any budget for furniture? If so, then a circle of swivel armchairs would work very well in your space and allow you to put the TV on a different wall.

Four small-scale armchairs on swivels arranged around a round coffee table:

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Two swivel chairs instead of a sofa with entryway behind the chairs:

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Here's another layout for a small space with no foyer.

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although 4 chairs look cool in those inspo pics, no one (homeowner) wants to hang out in an arm chair on a daily basis when they don't have company over.