Intermediate Fix and lateral safety margin.
 

Anybody know the width of the IF safety margin in NM? The Jepp mentions a lateral safety margin on 2.5 nm either side of the track for the IAF, and an extra 2.5nm secondary buffet margin.

It depends on the approach, is it straight in, NDB, VOR. For a VOR approach it will taper from the IAF to the IF in the primary area from 2,5 nm to 1 nm and the secondary area from 2,5 to 0,5 nm either side of track. NDB from 2,5 to 1,25 nm in the primary area and from 2,5 to 1,25 in the secondary area

If I am not wrong, the safety margin is usually 2.5 NM either side of the track, as you mentioned for the IAF.

Where is this referenced?

ICAO Doc 8168 Volume 2 Part II Conventional Procedures

And for a RNAV ILS approach? What is the lateral safety margin from IF to FAF?

2.5nm either side at the IF, and reduces linearly down until intercept of the ILS OAS contours. The width is variable depending on the parameters of the ILS but is typically about 1.5NM at the FAF and reduces further down to the MAPt.

I cant seem to post an image. But PANS-OPS Figure II-1-1-2 gives you a good idea of how it works

Alpha

https://cimg7.ibsrv.net/gimg/pprune....86aa8293e7.png