Sa'i on upper floors is valid according to the majority of scholars, as the floors are within the boundaries of Safa and Marwah — upper floors are recommended for elderly and families due to less crowding.
Yes, performing Sa'i on the upper floors of the Mas'a (Sa'i corridor) is valid according to the vast majority of contemporary scholars. The Saudi Arabian expansion of Masjid al-Haram added multiple levels to the Sa'i corridor to accommodate the growing number of pilgrims. These upper floors are constructed between the same two hills of Safa and Marwah, within the legitimate boundaries of the Sa'i path.
The permissibility is based on the principle that what matters is walking between Safa and Marwah within their boundaries — the vertical elevation does not change the fundamental act. This is analogous to performing Tawaf on the upper floors of the Mataf, which is universally accepted. Major scholars and the Saudi fatwa committees have affirmed the validity of upper-floor Sa'i.
Practically, the upper floors offer significant advantages: less crowding, smoother flow, better accessibility for wheelchair users, and a cooler environment. The green markers indicating where men should hasten their pace (raml) are present on all floors. The distance per lap is essentially the same regardless of floor level. For elderly pilgrims, those with mobility challenges, or families with young children, the upper floors are strongly recommended for a safer and more comfortable Sa'i experience.
Source: Saudi Permanent Committee for Scholarly Research and Ifta (al-Lajnah al-Da'imah)