If you keep it in US$ then you are engaging in currency speculation. Do you want to do this and if so then you need to find a US$ account to put it in. Otherwise you should just convert it to € and then decide what to do with it. In that case sounds like using it to fund your self build might not be a bad idea?
If you really need it for your house then you really don't have much choice but to follow Clubman's proposal and convert it to EUR immediately.
If I were you I'd leave it in USD. I have some US Shares and cash that I'd like to "bring home" but with the dollar the way it is I'm not going to do so. I don't need the money right now and I'm willing to speculate (that's what you do when you buy shares anyway).
The USD has been getting steadily weaker during the Dubya Bush years. Perhaps there will be a turnaround after the election in 11 months, should the Democrats get in. Perhaps the Republicans will retain power and the slide continue.
Your call, it's just like shares going up or shares going down.