The range between mesh nodes is generally one or two rooms through typical hollow (interior) wall construction materials. Solid walls (brick, concrete, plaster) will reduce this range. Exterior walls are much more variable. Metallic vapor barriers and aluminum siding will impact range significantly. Obviously, if those exterior walls are solid (brick, etc.) that will have a similar impact on reduced range. So, in general, getting coverage to outbuildings is not reliable using just mesh units. I typically recommend running an Ethernet cable in this case, and if you choose to go that route, there are a lot more issues to discuss regarding exactly how it needs to be connected. There are (relatively expensive) wireless products that can be mounted on each building to create a sort of "virtual Ethernet" connection, and that might be an alternative to running an actual Ethernet cable. But, setting these up and configuring them is probably not for the faint of heart, depending on how comfortable you are with networking and wireless details.
I would also just note that the mesh nodes use the fewest hops to get traffic where it needs to go. Since WiFi will slow down significantly to get through, this means even relatively distant mesh nodes will usually communicate directly with the primary node (the one connected to your internet service) rather than going through an intermediate node. This is why the placement advice for optimal performance and coverage is to have the primary as close to the center of your home as possible with secondary (wireless mesh) units one or two rooms away from there – close enough they can all get strong 5GHz WiFi connections directly to the primary. From there, they will provide 2.4GHz and 5GHz coverage to more distant WiFi client devices. I'm just pointing this out so you don't have unexpectedly poor performance when you try to extend your network.
Lastly, keep in mind there's a single 5GHz channel used system-wide. That channel has to carry traffic between access points and 5GHz WiFi clients as well as carrying that same traffic again through the mesh interconnect. So, there is a practical limit to how many total access points can be supported in a system without things actually getting worse (slower at best, intermittent connectivity at worst). I believe the current limit is 5.
In short, these systems are deceptively complex. Think about the constraints and plan any changes carefully in advance. Make small changes and test between each change. I hope some of this information is useful!
