Existing stair seating seimic gap  

Assuming you are referring to the SESOC Interim Design Guidance – which version are you using? I don’t think the current version contradicts C10, or NZS 1170.5A1 which is the basis for C10.

That aside – it should be noted that the SESOC Interim Design Guidance is intended for design, not assessment. While some parts of the guidance may be relevant to assessment, many parts would be overly conservative for assessment purposes.

It was from Version No. 9 – 26 March 2013, which I downloaded from the SESOC website.

There is nothing to indicate it has been updated, but it appears to be a draft. I checked the revised copy, which is the SESOC guideline 0.9 from September 2019, and it shows 1.5/Sp but is marked with a strikethrough. saying **content below now superseded by Verification Methods** 

SESOC Design panel joints to accommodate 1.5/Sp times the ULS
Recommendation: displacements.
It worth mentioning that  MBIE Practice Advisory 13: Egress stairs – provides the 2 * ULS drift

• Clearances and seatings for stairs should be capable of sustaining at least twice the Ultimate Limit State (ULS) inter-storey displacements (drift) as calculated in accordance with NZS 1170.5.

It is mainly to design a steel ledge support in an existing building 

Hi Khaled – thanks for the clarification. The SESOC website clearly identifies that versions 9 and 10 of the Interim Guidance are superceded, as shown at SESOC Interim Design Guidance Design of Conventional Structural Systems – SESOC.

The older versions are left online as a resource for situations where people are trying to understand design decisions made for older structures.

Thank you Nick for your response and clarification