Westminster Abbey emerged as the favourite venue for the wedding after Middleton, 28, was spotted leaving the church recently.
It is believed the couple are keen on a Mar 2010 date but may end up choosing a summer month when the weather is more dependable.
"They're not going to be rushed and want to take things at their own pace," Sky News quoted a palace spokesman as saying.
"They're very important decisions to make and like any couple they want to make sure they've properly considered all of the factors."
The spokesman added: ""They do have a specific venue and specific date in mind. They have given firm direction on costs. But they need to consult with all the relevant parties on this."