dhceramics is spot on Who ever says you don't need expansion joints is talking Bollocks. Your going to get some kind of movement in the floor over time. Some really good advice has been given here. Schluter make great systems and are top quality and you pay the price. I would leave the floor for 6 weeks from when it was laided by the builder, you could if you wanted make an expansion joint your self in the floor using an angle grinder and grinding a line where the two floors meet and place some silicon in there good qaulity silcon will go up to 25 - 30 % expansion, When you come to tile the floor make sure you well prime it and place either an expansion strip over the top of the exsisitng one or use silcon as suggested mapei do silicon to match the grout and you can't tell. Best of luck mate