Do I first need a Data Warehouse?

I'm currently supporting SQL based system that comprises of 700 plus sql tables.

The database size is small, only 2 gig of data.

The database is highly normalised.

The exiting reporting application is built in winforms and all reports are hand crafted in C# writing out to excel.

If a new report or tweak is required this requires a developer to make a code change and release a new version of the reporting application.   The takes up a huge amout of the deveopment resource.

Management now require dashboards and self service reports.

I've looked at various tools in the market and decided on Power BI.

I then have a number of options.

1.  Run Power BI against the existing SQL database.

2.  Copy data to a Data Warehouse each night and run BI aganst that.  The DW would just be a copy of the OLTP database structure.

3.  Create a Data Warehouse withthe data flattened out (i.e. transform the data into fact and dimensions using SSIS) and run Power BI against the new Data Warehouse structure.

Please could someone advise as to the beast approach.

Thanks.