Research into residential property investment has historically been limited, yet the UK residential market has an estimated value in excess of £2 trillion and is of key importance to the domestic economy. The purpose of this paper is to assess the potential of investment in the unsecuritised private rented sector to improve the efficiency of mixed asset or real estate portfolios, through the use of Modern Portfolio Theory (MPT). However, given the limitations and uncertainty of real estate data, including such issues as data collection methodology, asset illiquidity and indivisibility, sample selection, and appraisal smoothing, the data is 'bootstrapped' to produce a distribution of efficient frontiers to test the reliability of the results. The results suggest that the inclusion of unsecuritised residential property in the private rented sector within real estate and mixed asset portfolios is positive, but limited. The issues that are examined in this paper are of significant importance to real estate portfolio investment and allocation, and highlight the significance and potential for further research.