OK, its been a while and I do not have ICAO PANS OPS 8168 to hand....
However, for an ILS approach, terrain/obstacle separation is provided by a complex series of sloping surfaces (Obstacle Assessment Surfaces OAS) extending out from the surface (touch down area of the runway). Quite simply, if nothing penetrates these surfaces (or the ICAO Annex 14 surfaces) then operations to Cat III are possible so long as the ILS installation, lighting, surface signs, LVP protection etc are in place and to appropriate criteria to support such ops.
In the case of an ILS to Cat 1, with no terrain or other obstacle to affect, then the minimum (pilot/operator may require a heigher DH) decision height would be 200ft above the threshold. Normally, for a Cat 1 ILS, the controlling obstacle would be a B747 SP (tail height 20.06 metres) holding at the normal Cat 1 holding point at the RWY threshold at 45 degrees to the RWY centre-line (to get the tail as near to the RWY as possible). If a particular airport has taxiways other than just at the threshold, the procedure designer will have to check that a 747SP holding at these does not penetrate the OAS surfaces. The A380 may change this!
Remember, every approach may result in a missed approach, so obstacles/terrain have also to be considered for this event. If there are obstacles affecting the missed approach, then the decision height would have to be increased so that start of climb commences earlier on the approach to clear this (these) obstacle(s).
In terms of practical clearance of obstacles, it is assummed that the aircraft will remain within full scale deflection of the LLZ and GP. This will keep the acft safely within the OAS "funnel".
At decision height, well, it is quite simple. The assumed height loss margin, different for each acft category A thru' E (using a pressure altimeter for CAT 1 ILS so extra height factored in to cater for altimeter errors) is simply added to the obstacle to give an Obstacle Clearance Height (OCH) or altitude (OCA) and converted to DH/DA (by the operator)with the proviso that the DH will not, in the case of CAT 1 ILS (pressure altimeter) be less than 200ft.
For acft category A (light acft) versus category D (heavy acft) the height loss margin due to inertia is less. On an approach chart produced by the procedure designer you might see the following
Acft CAT A OCH 190
Acft CAT B OCH 199
Acft CAT C OCH 209
Acft CAT D OCH 227
The DH itself calculated by the operator. DH could equal OCH, so long as it is not less than 200ft of course, but in practice many operators/administrations may add a buffer to this to cater for acft type/performance/crew experience etc.
At DH then, decide! All you have between you and the obstacle is the assumed height loss margin! And don't forget, although buffers to allow for pilot reaction time and acft reaction time to commence climb are calculated and factored into the OCH, if the controlling obstacle for the approach is in the missed approach area you need to start climbing! Obstacle clearance for the mapt case (climb gradient 2.5%) is only 98ft for the initial mapt phase! But that is another story.....