I try to put anything that is flammable or smelly in a separate structure from a home.

Many years ago we had friends who had their garage just as you describe. Their daughter was cutting the grass, ran out of gas, and pushed the mower into the garage to refill the mower. Spilling gasoline onto the hot mower, she started a fire that burned their house to the ground before the fire department could arrive. In addition, there is always the danger of someone starting the car and, having forgotten something in the house, leaving the engine running. The carbon monoxide is a killer.

My neighbors and FIL did this. The neighbor's 4 walls are poured concrete. Their house is also made of metal studs.

My FIL put it right under his master bedroom. He thinks he can leave it unfinished but I'm betting it's going to cost him a pretty penny to bring to code so it's sealed and won't let CM into the main floor.

Our shop will be out back.

Even if not required, the house-to-storage area should have a fire-rated door with a substantial self-closer. Your architect/designer/builder should know of what is required to meet local Codes. Though likely not required, a small exhaust fan in the storage area/garage is often recommended for better indoor air quality.

Thanks for all the tips. Most people have attached garages adjacent to living areas and don't die from carbon monoxide or commonly have fire loss, that's why I was asking what I would need to do to insulate it from the rest of the house. I would never park a car in there, probably wouldn't even have accessible driveway to it, but I understand all the concern. Maybe will just put up a small out-building instead. Thanks!

The door from the garage into the other areas of the house should be a nice solid door that won't let spiders in. For the exterior door into the garage area it might be better to have a large carriage house door or garage door so you don't have a slider track on the floor, dirty and difficult to wheel over. Also with a sliding glass door, even if you put in a sliding door in that has a top mounted track, you might be required to put in an exhaust fan.

We are having this in our new build. There are applicable codes that have to be considered to have a garage under living space but it's doable. Your builder can advise.

I don't understand the resistance to attached garages. We have lots of rain and freezing winters and I want to drive in and get into my house without facing elements. but that's me.

I wouldn't want a garage in the basement if it was the only garage. [think bringing groceries in and having to walk through the basement up the stairs etc.]

If it's just for gardening supplies lawn equipment etc. I would think you could just treat it like a walk out part of a basement?

In our current home we have a workshop in what we call a cellar [only accessible from yard but It has double doors]. It has the water heater furnace fuse box storage and all hubs' tools and a workbench etc.

We keep the lawn mower in the attached garage which is on the front of the house though as we usually start mowing in the front [might be because mower is there ha].

It is just personal bias due to people either not living in an area with those house features or they simply don't like them.

For example basements in CA. People who don't live in CA, people in CA who live in homes without them or have never been in a CA home with a basement, will tell you no one does basements in CA due to earthquakes. That is a completely false conclusion. In my area of Los Angeles, land costs are so high, that maximizing costs and size of the project with a basement is very common. Probably 70% of tear downs are rebuilt with a large basement.

I would also never buy a house without an attached garage, but would also never buy a house with the only garage being in the basement. Not because of CO death or fires, but because it is a PITA to carry kids, groceries and yourself up those stairs several times per day! Did it with our first house and vowed never to do it again.