But again, the devices will relay the message from one to another, so it’s OK if the hub is 40 m away as long as there is a mains powered device that can repeat the message halfway in between the original device and the hub. This graphic is from the blog article that @viguera already linked to:
In a European home, a typical repeating device would be a plug in pocket socket or in wall relay. In a US home, it’s often light switches.
If the interior walls are cement, brick, or Adobe, then The range will be much shorter and may not be able to get through the wall at all. In those cases you have to essentially bounce the signal out through the doorways and down the halls.
The following thread discusses more about how to improve coverage. It is talking about zwave, but the same principles apply to Zigbee.
Although your home is smaller, the fact that you have brick walls means you have to deal with the same kinds of issues as someone with a very large house.
I don’t own any Hue devices, but most community members highly recommend connecting Hue devices to the Hue hub and not directly to SmartThings. If you are also doing that, since they are part of the Hue hub network, they will not repeat ZigBee messages from SmartThings.