Dear fellow DIYers

I am hoping to move a bathroom from the front of my house to the
middle, but I don’t really want to have to put a new soil stack in
unless necessary. My question is: How far is too far in terms of a
horizontal run for a soil pipe? I am proposing to have an (almost)
horizontal run for about 3 metres. I’m not too bothered about how it
will look as it will run along the ceiling of an integral garage. I
know that they have to have a certain fall distance per metre, I’m
just not sure what it is. I have searched the group but it seems like
older posts have gone? I would appreciate any advice on the matter and
no, I am not going to install a macerator - they are the devils work!

The house is semi-detached and luckily the side where the new bathroom
is to go is the farthest outer wall, so it would not be the end of the
world if I had to dig up outside etc.

Thanks in advance.