If the cable (cat5 LAN/ethernet) is less than 100 meters (328 feet) then the speed will be same (100 MBit/s) as if you were to use a 1 foot cable to the modem's LAN socket. Which more than enough for any DSL connection.
Although if your Modem only has one LAN output socket (port) then your will have to unplug your Mum's (Mom's) internet cable while yours is connected.
Emm...correct me if I'm wrong here, but isn't drilling two holes in the floors of both rooms a bit pointless? For starters, if you're both on the ground floor, you'll likely be drilling into solid concrete (i.e, the foundations of your house). If your builders were clever (or if your house is newer than mine, quite likely), you'll still encounter the problem below.
Even if you're both on the first floor, you'll be drilling into the recess between the floor joists and the ground floor ceiling. This means you'll be able to get the cable into one hole just fine, but have no way of making sure it comes out of the other one
There are special tools and ways of "fishing" to do exactly this but its not always an easy job.
One of the best tools for the job is a wire coathanger provided the holes are close enough and in the same void between joists, you need a special sixth sense sometimes to visualise where the exit hole is
basement has no ceiling, so it is just the floor...the foundation is not that close to where i would dril, and there already is a hole there (i just have to expand it a little bit).
If you have a basement with no ceiling (as in, you can see the joists and bearers of the ground floor), it should be perfect to go under the floor then Most houses over here (well, about 95%) don't have basements, the ground floor is just built straight onto the foundations
100 base UTP (common CAT5, what you should have [unfortunatly, both ISPs I've used supplied patch cables instead...probably what your ISP has given you too if they told you 10m max]) is good for 100 metres. If you're using patch cables (*shudder*), the max recommended length is 6 metres. But you could theoretically have an infinite cable length if you put a hub, switch or router every 100m to "regenerate" the signal
I live out in the country where most houses are bungalows built in the 80's/early 90's. We're lucky to have indoor plumbing and electricity We have a garage that comfortably fits an MGB (quite long), but it's an external building out the back Agreed though, most town houses or housing estates have ridiculously small garages