Since a large part of buildings around the World and especially in Northern Europe, were built without or with insufficient seismic provisions, seismic assessment of existing buildings is a critical and endless issue to be solved by earthquake engineering [1]. Cost-benefit approaches for the assessment and retrofit of existing buildings in Switzerland started to be used on a day-to-day basis in 2004 with the Prestandard SIA2018 [2]. This Pre-standard provides guidelines on the measures to undertake to improve the seismic safety of a building taking into account the seismic risk, and in case, if retrofitting measures are commensurate. The results provided by the prescribed method are, however, quite rough, and the effects of an eventual retrofit needs to be more finely quantified. Different seismic retrofitting measures have been proposed for unreinforced masonry (URM) buildings. Examples are: adding sandwich columns to partition brick walls [3], jacketing of columns, adding structural walls, and construction of a mat foundation [4], reinforced cement jacketing of the main load carrying walls [5] and using FiberReinforced Polymers [6,7]. The effect of those retrofitting measures has been assessed using a variety of methods including Finite Element numerical modeling [1,8] and vibration measurements [9]. One of the common measures to retrofit existing URM buildings in Europe is to create composite slabs by adding a reinforced-concrete (RC) layer over the existing wooden floor or even to replace it by a RC floor. This measure is generally proposed to improve the comfort and the soundproofing of the building, and is thought to be beneficial for the seismic behavior, as well. As a result, the masonry walls are linked together creating a diaphragm effect, which also prevents out-of-plane collapse of the walls [10,11]. This paper presents the evaluation of the retrofitting measures on a typical residential building in Switzerland [12]. The studied building is The method to assess the retrofitting work is based on two complementary techniques. Several ambient vibration recordings were performed before, during and after the retrofitting work in order to monitor the evolution of the dynamic behavior of the structure. Moreover, numerical models representing the state of the building before and after the retrofit work have been developed to perform nonlinear dynamic analyses using various ground motion records. Changes in the dynamic behavior should prove the effectiveness of the retrofitting, especially a change in the diaphragm effect. Using ambient vibrations, properties of the soil and features regarding soil-structure interaction can be evaluated, as well.