The base of the palace would be more than likely the Grand Canal entrance to a palazzo. In a letter from Janet Jewell (bellow), she thinks Sargent has painted is of the Palazzo Grimani. All of these grand palaces have their front entrances onto the canal instead of a paved street. Here, Sargent seems to be just interested in the base of the columns and this oriental pattern along the foundation. 
 
 

 
Palazzo Grimani 
c. 1904 
Watercolor 
 

Other Sargent paintings of canal entrances: 

 
Entrance to a Venetian Palazzo 
1906-10 
Watercolor 

Notes: 

• See Overview of Venice
• See the year in review  1904

From: Janet Jewell
janet s mje well @ hotmail. com  
Date :  Wed, 03 Apr 2002  

Dear Natasha  

I live in North Devon, in England, and have just been browsing through your website (which I have enjoyed)  . . .  

I have been greatly interested in John Singer Sargent's work for a few years, the first work I saw of his was in York Art Gallery in Northern England.  It was a charcoal drawing of a beautiful woman, whose name escapes me, and from that moment on I was hooked.  I went to the exhibition at the Tate Gallery in London in 1999 and was absolutely overwhelmed.  One of my favourite watercolours is 'Base of a Palace' painted in 1904 in Venice, which is possibly the base of the Palazzo Grimani. I went to Venice in 2000 and found the Palazzo Grimani with every intention of photographing the building, but unfortunately it was being restored and was covered in tarpaulins.  I was very disappointed.  

Yours sincerely  

Janet Jewell  
 
From: Natasha 

I loved your letter. I found an image of Palazzo Grimani. It's not the best but it helps.  

There are three arched opening at the front facade - a larger middle one and two smaller ones which flank it (I'm not sure what you call them). It appears Sargent is painting the base of the steps of the far right opening and is capturing the base of that one window. I think you correct about this Palazzo. It looks very much the same.