It's probably way cheaper to soundproof* a home office, than to buy a larger house. Speaking from personal experience.
* "Soundproof" is the commonly used term, but obviously that's not realistic if taken literally. In this context, I use "soundproof" to mean virtually eliminating all psycho-acoustically relevant sounds from outside the room.
Sounds like a tough situation. I've had some success with my own house which is only slightly larger. I'm happy to DM if you want to bounce some ideas around.
Here's the simplified version, but I'm happy to give more detail.
Starting point:
- Raised ranch in New England, circa 1960's. (Not in an earthquake zone.)
- Electrical service: 200 amp service from road to house, but 100 amp panel.
- Semi-finished basement, not very compliant with current building code.
- Crappy basement corner office, about 12'x 12', with 1 egress window and the house's electrical panel.
- Basement occasionally got wet from heavy rains.
Preliminaries:
- Electrician friend upgrades electrical panel to 200 amp service: several hundred $ (USD).
- Hire company to add interior drain (under slab) in vicinity office corner, and add a sump well: $3k
- Hire contractor friend to install one more Lally column under the house's main beam, to satisfy building code requirements.
- Rent a storage pod for outside the house, to hold all the basement junk that would be in the way during construction.
- Building & electrical permits: < $100 (my town is awesome)
- Hire company to install Mitsubishi mini-split heater/cooler. 1 inverter in the office, 1 in another room. $7k. (This actually happened after the drywall was up.)
Major steps for the office soundproofing:
- Install subflooring over the basement concrete slab. I used 3/4" Advantech from Home Depot, and fixed it to the floors with 1/4"-diameter TapCon screws.
- Re-frame the office walls with double-framing.
- Install steel hat-channel on joists in ceiling. Ceiling drywall will hang from this.
- Install 14-2 NM wiring for wall- and ceiling-outlets.
- On walls and ceiling, drywall is two layers of 5/8" Type X drywall, with Greenglue sandwiched in between.
- For the window, I bought a pre-cut piece of plexiglass (I forget exact thickness, maybe 3/8"), and got clever with strips of neoprene rubber and some hardware to hold the plexiglass in place. Plexiglass was about $100 IIRC, but probably cheaper if it didn't get shipped from the West Coast.
- For the doors, I used pre-hung solid-core doors. There are two doorways: one for accessing the electrical panel, and one for leaving the room. Each doorway uses two doors, spaced about 8" from each other. I used various sheets of MDF and plywood to add mass to the doors, and Greenglue sandwiched in between. I also used various tricks with neoprene gaskets and strong magnets to get a good acoustic seal on each door.
- A soundproof room tends to be airtight as well, so you really need forced-air ventilation. I installed a 100 CFM inline fan, which I think was overkill.
- Airflow into / out of the room is provided by two "dead vents". Basically, air ducts with acoustically useful baffles. Surprisingly effective combination of high airflow but minimal sound transmission.
- I paid other people to tape the drywall and paint the office. But really they did the whole finished area in the basement, so I'm not sure what just the office would have cost.
Costs and other considerations:
- Rental of a storage pod can stretch out longer than you intended, because these projects can go slowly.
- At some point you'll estimate the number of fasteners you'll need (drywall screws, TapCon screws, etc.) You'll assume you made a math error because the number is 10x higher than your intuition suggests. The good news is you're correct; the bad news is that you'll actually use 20x more than your intuition says.
- Soundproofing an office may require a building permit, which may in turn require that you bring the construction area up to current building / electrical code.
- Having friends in the construction business is invaluable. They're your best source of unbiased advice.
- There's a ton of conflicting information about office soundproofing out there, and just as many gimmicky products. You'll need to do a lot of homework.
- Many building contractors optimize their techniques / designs for maximum speed and minimum cost. Good soundproofing requires somewhat different designs. You may need to do some construction steps yourself to make sure they're done right.
- Unless you already have construction tools, you'll probably end up buying some yourself, and you'll need somewhere to store them.
* "Soundproof" is the commonly used term, but obviously that's not realistic if taken literally. In this context, I use "soundproof" to mean virtually eliminating all psycho-acoustically relevant sounds from outside the room.