A study was made of radon-safe building in 300 Finnish low-rise residential buildings using data obtainedfrom a questionnaire study. The study also aims at finding the main defects in design andimplementation and how the guidance given on radon-safe building in slab-on-grade houses has beenfollowed. According to the guidance, the prevention of the flow of radon-bearing air from the soil intothe house is recommended to be carried out through installation of aluminized bitumen felt and use ofelastic sealants. Second, as a precaution perforated piping should be installed in the subsoil of the floorslab. This piping will be connected to a fan if the action level is exceeded, providing an active sub-slabdepressurisation system for the house.The median indoor radon concentration in the houses was 305 Bq/m3. This is 32 % lower than themedian of the estimated reference values. The action level of 200 Bq/m3 was still exceeded in 40 % of thehouses. In most houses with slab-on-grade the prevention was based only on the installation of a sub-slabdepressurisation system. Sealing was performed in a low number of houses. In 80 % of houses with asub-slab piping connected to an operating fan, radon concentration was below the action level of 200Bq/m3. In houses with piping but no fan, the corresponding fraction was only 45 %. The correspondingmedian values of radon concentration in these houses were 55 and 220 Bq/m3. Sub-slab piping without afan had no remarkable effect on radon concentration. In houses with crawl-space and edge-thickened slab,radon concentrations were low. The choice of foundation system thus significantly affects the indoorradon concentration.Performing the sealing work is an essential part of radon-safe building. However, sealing measures werecarried out in about 30 % of the houses and in many of these houses the work was not carried out alongthe guideline. However, through performing careful sealing work, good results were achieved withoutthe activation of the sub-slab piping. The importance of complete and careful sealing work should bestressed in advice and guides concerning radon prevention.