A Complex Methodology for Quantifying the Contribution of Informal Construction to Structural Failure Risk in Nigerian Cities: Theory, Data, and Policy Implications

Informal construction, characterised by the absence of formal design, permitting, and supervision, constitutes a majority of the urban building stock in Nigerian cities and is widely held to be the principal driver of structural failure risk, yet the evidence base for this assertion remains largely anecdotal and the mechanisms through which informality increases structural risk have not been rigorously theorised or quantified. This study develops a complex methodology for quantifying the contribution of informal construction to structural failure risk in Nigerian cities, drawing on theoretical, empirical, and policy analysis dimensions. A theoretical model is developed that conceptualises informality as a multi-dimensional risk amplifier operating through design quality, material quality, construction quality, maintenance regularity, and regulatory non-compliance pathways, each of which modifies conditional failure probabilities relative to formal construction. The theory is operationalised using a Bayesian network risk model populated with data from structured field investigations of 1,450 buildings across six cities representing all six geopolitical zones, including Kano, Enugu, Maiduguri, Ibadan, Port Harcourt, and Ilorin. Structural failure probability for each building was estimated using a rapid structural assessment protocol, and informality level was characterised using a composite index of seven observable indicators. Results indicate that highly informal construction is associated with a mean annual collapse probability 8.6 times higher than fully formal construction after controlling for building age and use class. The relationship is nonlinear, with the greatest risk amplification occurring at the transition from partial to complete informality. Policy implications include the derivation of an evidence-based threshold for which informal buildings pose greatest societal risk and the estimated cost-effectiveness of targeted regularisation programmes. Keywords: informal construction, structural failure risk, Bayesian network, Nigerian cities, building regulation.