Taiwan is among those countries or places of a developed economies such as Hong Kong, Italy and Greece where unauthorized building works (UBWs) are prevalent. A variety of measures have been employed in various countries to tackle the UBWs that include at least immediate teardown, pardon, amnesty through retrospective penalty and etc. In response to the accumulated massive number of illegal extra floors or extensions on the rooftop, or unpermitted enclosed balcony, a set of rules have been established in Taiwan to identify, and record UBWs for them to be later dealt with. However, due to historical, cultural and even political factors, a significant number of UBWs still remain untouched. Official statistics show that as of Oct. 2017, there was 670,000 UBWs in Taiwan nationwide. That is to say, approximately 8% of households committed construction of UBWs in their houses, apartments or condos. There is no exception in the largest city of Taiwan, Taipei where the laws are widely recognized to be well enforced compared to other Taiwan cities. Despite the seemingly uncontrolled situations in Taiwan, prior to demolition, UBWs are subject to annual house taxes. There are at present over 300,000 UBWs registered in house tax records in Taipei alone. Because land and buildings are taxed separately at differing rates, UBWs can largely be viewed as a special form of buildings as far as house tax is concerned. UBWs are taxed annually along with the legal buildings which they are attached to. In spite of the long practice of taxing UBWS, the rationales and revenue contribution of this tax is rarely discussed in academic literature. In addition, how the value of UBWs is determined for taxation is poorly understood. This paper first reviews the pieces of legislation passed and amended over time that regulate UBWs. This historical account is hoped to understand the rationales behind the legal changes. We afterwards carefully examine the tax records that comprise of both legal buildings and UBWs. Various regression models are employed primarily to unveil the relationship between house taxes on the legal buildings and UBWs. In so doing, how the UBWs is assessed compared to the legal buildings they are attached to and their relative tax burden will be revealed. Those empirical evidence allows us to better interpret the house tax on UBWs either as a supplementary source of tax revenue, a penalty on illegal activities or a tacit acknowledgement of their de facto existence.