We model the carbon recombination line (CRL) emission from the photodissociation region (PDR) surrounding the ultracompact (UC) H ii region W48A. Our modelling shows that the inner regions (A_V ∼ 1) of the C ii layer in the PDR contribute significantly to the CRL emission. Using models spanning over a large range of parameters that are typical for the environments of UCH ii regions, we explore the dependence of the line ratios of CRL emission on the density of the PDR and on the far-ultraviolet radiation incident in the region. We find that it is possible to constrain the density of the PDR by observing a suitable set of CRLs. If the neutral density in the PDR is high ( ≳ 10⁷ cm⁻³), then the CRL emission is bright at high frequencies ( ≳ 20 GHz), and the absorption lines from such regions can be detected at low frequencies ( ≲ 10 GHz). By modelling CRL observations towards W48A, we can show that the UCH ii region is embedded in a molecular cloud with a density of about 4 × 10⁷ cm⁻³.