TLDR: You needed to keep a document establishing that you had undertaken to make the investment no later than 31 December 2012. The transitional measure concerned acquisitions made from 1 January to 31 March 2013.
What you needed to substantiate
You needed to be able to establish that your undertaking to make the Scellier investment had been made no later than 31 December 2012.
The supporting document kept needed to link this undertaking to the acquisition of the property in a future state of completion.
Acquisitions covered by the transitional measure
The transitional measure applied to acquisitions made between 1 January and 31 March 2013.
To determine the acquisition date, you had to use the date on which the authentic purchase deed was signed.
Consequence in the absence of supporting documentation
If you could not substantiate the undertaking made no later than 31 December 2012, the Scellier tax reduction could not apply to the acquisition concerned.