Possible Clues:

• Extra accommodation block
• Extra wing (on building)
• Sounds as if a throat's what the girl may take to the river
• Extension built by queen to house old flame
• It's built on a northern river
• Has it been added to follow a double you, by the sound of it?
• Addition to building or document
• A letter, say, may be an attachment
• Side building
• Addition to document