http://www.tlc-direct.co.uk/Book/4.4.1.htm
-Guide to the 16th Edition IEE Regulations 4.4 - Cable supports, joints and terminations:




Cabling in floors:
(insert reference here, please someone who knows!)
Where cables are run beneath boarded floors, they must pass through holes drilled in the joists which are at least 50 mm below the top surface of the joist. This is to prevent accidental damage due to nails being driven into the joists. The hole diameters must not exceed one quarter of the depth of the joist and they must be drilled at the joist centre (the neutral axis). Hole centres must be at least three diameters apart, and the holes must only be drilled in a zone which extends 25% to 40% of the beam length from both ends.

Also: 522-06-05 “A cable passing through a timber joist … (e.g. under floorboards), shall: i) be at least 50mm measured vertically from the top, or bottom as appropriate, of the joist or batten, or ii) … be protected by enclosure in earthed steel conduit securely supported, or by equivalent mechanical protection sufficient to prevent penetration of the cable by nails, screws and the like”

Also hole centres must be at least three diameters apart.

Notches must be in a zone that is from 10% to 25 % of the beam length from both ends. The depth of the notch must be no greater than one eigth of the joist depth.

Holes seam to be the accepted way to do it now, but in many houses there are allready notches where this is the case used safe plates should offer the required protection: tlc safeplates Screwfix safeplates-can be had from other electrical retailers also.This forum topic shows what happens when you dont use them

here's a couple more links Ive just found that include infomation about notching/drilling joists, when i get chance I'll extract the infomation
(anyone else please feel free to do it)
http://www.nhbcbuilder.co.uk/NHBCpublications/LiteratureLibrary/Technical/filedownload,21418,en.pdf http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_A_2004.pdf

(note i appreciate this info is currently all over the place, please feel free to neaten it up. i added the floor infomation so that it is recorded as i had searched for it for my use and figured this is what the wiki is for! -sm1thson)

See here for concealed cables in walls