A Parametric, Geometry-Aware Residential Construction Cost Estimation Model for Ghana: Design, Validation, and the "Completeness Gap" in Informal Contractor Quotes

Ghana faces a residential housing deficit of two million units. A key driver of project failure is the "completeness gap", a systematic discrepancy between informal contractor quotes and actual costs. Informal estimates often use flat per-square-metre pricing that omits essential structural and finishing components, leading to project abandonment mid-construction. This paper validates a parametric, geometry-aware cost estimation model via the GhanaHousePlanner (GHP) platform. The model provides self-builders with itemised bills of quantities (BoQ) reflecting the true cost of code-compliant construction in Ghana. The GHP model uses seven calculation modules: foundation, blockwork, cement, structural steel, roofing, plumbing, and electrical. It features a primary geometry-based mode and a formula-based fallback. Accuracy was tested using three case studies (75, 120, and 200 per-square-metre homes) benchmarked against February 2026 market prices in Greater Accra.GHP estimates (GHS 519,000 to GHS 1,398,000) were 29 to 98 per cent higher than typical informal quotes. This gap arises from the omission of structural steel (Y16 rebar), plastering, floor screed, and full services in informal estimates. Findings confirm that per-square-metre rates rarely cover the requirements for a fully completed, code-compliant building. The GHP model offers a transparent, auditable alternative to informal quoting. Despite material price volatility and labour market informality, the tool provides a framework for improving cost predictability and reducing project stalling in the sub-Saharan African housing market.