Help build a system

I have a tri-level home about 1500 square feet. My plan is:4 entry door sensors all on the ground level.3 motion sensors, (basement, ground level, and 2nd level).1 flood/freeze sensor in the basement.3 First Alert Z-wave smoke/CO detectors, (basement, 2nd level, and 3rd level).1 Keypad1 Base Station1 Range ExtenderI would locate the base station on the 2nd level aproximately in the center of the structure. This would be the closest point to one entry sensor, one motion sensor, two smoke detectors. All of these devices would be within 10-20 feet of the base stationThe range extender would be on the ground level, closest point to the other devices; (3 entry door sensors, 2 motions sensors, 2 smoke/CO detectors, 1 Flood/Freeze detector, all within 10-20 feet of the range extender. My concern is 2 of these devices will be located in the basement so the signal path is between floors, with metal heat ducts. When I look at the devices on the product pages it says 250 foot range - line of sight.Maybe this is a dumb question but what is meant by 'Line of sight'?Does that mean each device has to have an unobstructed 'line of sight' path to the base station or a range extender? No walls or floors obstructing line of sight?Thanks for any advice.