If you are working on sufficiently complicated projects it is inevitable that something like this will eventually happen. The way I have handled this in the p

If you are working on sufficiently complicated projects it is inevitable that something like this will eventually happen. The way I have handled this in the past is to hire a sub-contractor. Communication is key, especially if there could potentially be delays. I'm up front with clients in that I will typically discuss this possibility before begin working together. My basic contract has some language in it as well that essentially is saying that they are responsible for any fees that may be incurred by sub-contractors at the same rate we have agreed on. The gist of it (and it is clear to my clients from the start) is that I have the leeway to hire specialists as needed without their consent so long as I'm only billing out at the rate we have already agreed upon. If I'm unable to do this, then I will have a discussion with the client and they would need to sign a contract amendment at the agreed upon rate. So in general I will hire the sub at a rate that is less than my rate and actually make some profit off of this. The sub contractors I work with know I do this and find this a fair arrangement as it is a sort of "finders fee". The reverse situation has happened with some of these exact same people where I was sub contracting to them on an area where I had more expertise.