Lateralised overgrowth (LO) is characterised by the asymmetric increase in the size of any part of the body exceeding 10% compared with the unaffected contralateral one. LO is a key feature in various syndromic overgrowth disorders, such as Beckwith-Wiedemann spectrum and PIK3CA-related overgrowth spectrum (PROS). However, it can also present as isolated (ILO). Defining the aetiology of LO is critical due to the clinical implications and management strategies required for each condition. This report presents two patients who were followed up throughout childhood for ILO and were ultimately diagnosed with PROS through molecular analysis on DNA extracted from a skin biopsy, revealing the PIK3CA:c.263G>A (p.Arg88Gln) variant at a high variant allele frequency. This variant has been described in association with macrocephaly-capillary malformation syndrome but not with ILO. In conclusion, this is the first report of patients harbouring the (p.Arg88Gln) variant with a diagnosis of ILO, thus, highlighting the importance of considering ILO within the PROS and underscoring the necessity for somatic DNA testing. An early and accurate molecular diagnosis is crucial for guiding appropriate clinical management in order to ensure access to targeted therapies, emphasising the need for further research to refine diagnostic criteria and testing recommendations for ILO.