I've come across both these words recently and they seem to both translate to "infrastructure". Are they completely interchangeable or is there a difference in meaning? Or is it a question of regional usage or informal/formal language? Thanks in advance.
基建 is the short version or abbreviation of Capital Construction ( 基本建设), like log cabin construction or construction of sewer.
基础设施建设 means infrastructure, like hotels or transport.
基础设施建设，还没有简称，英语是Infrastructure 基础结构[设施](尤指社会、国家赖以生存和发展的, 如道路、学校、电厂、交通、通讯系统等),着重强调其“基础”的作用，和基本建设概念不同，是基建的，不一定是基础设施，比如建造俱乐部楼房（我个人理解）就不是一个城市赖以运转的基础设施了。