In principle, the informal sector should be included in national statistics. However, due to lack of source data, it is often excluded unless a particular effort has been made. For the countries that make adjustments for the informal sector, revisions are done by the central statistical office. There is a publication by the United Nations Economic Commission for Europe (UNECE) presenting an inventory of practices of 29 UNECE member countries in measuring non-observed economic activities (underground, illegal, informal or undertaken by households for their final use). The publication is “Non-Observed Economy in National Accounts,” United Nations Publication, Sales No. E.03.II.E56, ISBN 92-1-116882-1.
The OECD in 2002 issued a manual on measuring the non-observed economy.
The IMF web site also has some information on this topic: General Data Dissemination System
Some countries do include the informal sector in their national accounts estimations.You may also find the following paper on unofficial economy useful: Regulatory Discretion and the Unofficial Economy Dataset