I think it was like the official report: A communist burned the Reichstag.
Firstly: The KPD said it soo often loudly on the streets that if the NSDAP or Harzburger Front wins then they will burn the Reichstag, because they said its better to burn him than let “Nazi” sit in it.
Secondly: The normal Police of the weimarer republic got the fire raiser.
Thirdly: The Führer doesn´t needed to burn it, because he had the power above germany.
Fourthly: It was totally unnecessary and the Führer wouldn´t let a historical building (a prestige object and a symbol for united germany) burn down. He even started later a renovation of the Reichstag.