One of the central roles of legislatures is to scrutinize and amend legislation, but little academic attention has been given to the level of amendment which draft laws receive. It is not known, for instance, how much of the text is amended during the passage of a typical bill. This question is of more than academic interest. While draft legislation is subject to pre-legislative consultation, amendments introduced during the parliamentary process may receive rather less scrutiny. This paper describes a novel method to map and record the changes that take place in UK legislation as it passes through the parliamentary process. In a pilot study of 56 UK government bills from three parliamentary sessions since 2008, we found that on average about a third of the lines of legislative text were amended (running in some cases to thousands of altered lines) and bills increased in length by an average of forty per cent. Whether this degree of alteration is 'too much' is beyond the scope of this study, but it demonstrates the extent to which the final legislation may differ from the initial draft.