Position determination of smartphones in buildings is of growing interest. Within the scope of this study the position of a mobile user is determined on the basis of (tri)lateration. Four methods for the conversion of the received signal strength of the WLAN signals into ranges are introduced. In the first method, the so-called one-slope model, this conversion is performed using a logarithmic function. In the multi-wall model damping effects of the walls are considered, in addition. In two novel differential approaches temporal variations and various propagation effects are minimized by using reference stations. The first differential approach is based on DGPS where corrections from the reference stations are derived and applied by the user. In contrast to the first method, a differentiation of the received signal strengths based on the VLBI principle is performed in the second differential approach. In practical use cases conducted in an office building with three different smartphones, static positioning yielded to position errors of about 5 m in average under good spatial conditions.