Floor slab construction strongly affects both the cost and the on-site duration of low-rise buildings, yet transparent project-level comparisons for small boarding houses remain scarce. This study compares the direct cost and on-site construction duration of conventional cast-in-place and precast floor slab methods for the 50.36 m² second-floor slab of a two-story type-135 boarding house in Pugeran, Yogyakarta. A quantitative comparative case study was applied using project drawings, an itemized quantity take-off, the Indonesian Unit Price Analysis (AHSP), documented local price bases, a supplier quotation, and labor-based duration calculations. The conventional alternative cost IDR 35,746,962.53 and the supplier-inclusive precast alternative IDR 30,216,000.00, a project-specific saving of IDR 5,530,962.53 (15.47%), while the calculated on-site duration fell from 11 days to 2 days, a reduction of 9 days (81.82%). A sensitivity analysis shows that the cost advantage reverses once the precast price rises by about 18%, identifying the condition under which the conventional method becomes cheaper. The reported savings apply only to the prices, crew composition, and work scope adopted in this case study; they exclude factory lead time and curing, and they do not constitute structural verification of the precast system.
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