BY LEE YEN NEE -20 JUNE SINGAPORE — While “shoebox” flats have generally been attractive investments given their higher rental yields compared with other properties, analysts are warning that this may soon cease to be the case.Typically defined as units with a maximum floor size of 50 sq m, they have delivered an average gross rental yield of about 4.58 per cent so far this year, well above the 3.68 per cent that non-shoebox apartments rake in, according to data provided by Singapore Real Estate Exchange (SRX). “The main dwellers of shoebox units are singles, couples without kids, and foreigners,” said OrangeTee Head of Research and Consultancy Christine Li.This group usually prefers to live in or around the central areas, where most of them work and socialise, and they are willing to sacrifice space in return for location.However, a lot of the coming supply of shoebox units is concentrated outside the central area, with about 40 per cent of the 7,952 units currently under constructi