At present a 10 m prototype interferometer facility is being set up at the AEI Hannover. One unique feature of the prototype will be the suspension platform interferometer (SPI). The purpose of the SPI is to monitor and stabilize the relative motion between three seismically isolated optical tables. The in-vacuum tables are suspended in an L-shaped configuration with an arm length of 11.65 m. The design goal of the SPI is to stabilize longitudinal differential displacements to a level of 100 pm between 10 mHz and 100 Hz and relative angular noise of 10 nrad in the same frequency band. This paper covers the optical layout, signal processing and design aspects of the SPI, e.g. cross-coupling between the different degrees of freedom and fibre pointing noise are investigated. A simulation is presented which shows that with the chosen optical design of the SPI all degrees of table motion can be sensed in a fully decoupled way.