We have a 4 bedroom house which currently has 4 double bedrooms of varying sizes. DH and I have a good sized double with dressing rooms and ensuite, there is a large family bathroom and DS and DD have double rooms (although ds's is significantly bigger), the spare room is a small double. Ds's room has 2 windows and is next to the the bathroom, which is currently being renovated.
I mentioned to the plumber/builder that we could do with another bathroom due to DS and DD both taking ages in the morning and according to him we could partition DS's room and make a fairly decent ensuite bathroom. It wouldn't be particularly difficult as all the main bathroom plumbing connections are just the other side of the wall. He thinks it would cost around £500 for a partition wall and then roughly £2k for the tiles, shower, toilet and fixing it all. So £2.5k for everything.
After partitioning ds's room he would still have a double sized room of around 11'by 11' which I think would be plenty of room.
So would you do it and add the ensuite? I can't see that we would lose out as the bedroom would still be large enough and we'd have an extra bathroom hopefully relieving the morning pressure and adding a bit of value to the house. DH is non committal and thinks it's a lot of work and the bigger bedroom is a better feature....