The guest room should not be constructed in the southwest direction of the house because it is meant for the owner (Head of the Family) only.
You can plan the guest room in the northwest direction of the house. It is a clear and suitable place for the guests to stay in your house.
You may plan to construct the guest room in the southern direction as well. However, it will be better to consult a Vaastu expert before choosing this direction.
You can place or plan the guest room at agneya corner (southeast corner) also. There is nothing troublesome with this direction.
The door of the guest room should follow Vaastu guidelines as well. Northeast door, facing east direction; Northeast door, facing north direction; Southeast door, facing south direction and Northwest door, facing west direction are the most preferred choices.
If the guest room is situated at vayavya corner (northwest corner) or agneya corner, you can place the bathroom for this room at Nairuthi corner also, but don’t forget to take advice of an experienced Vaastu consultant beforehand.
If the guest room is situated at vayavya corner, you must provide window at northeast corner, towards the north direction, or in the west direction.
If the guest room is situated at agneya corner, you need to keep a window at south or southeast direction. Nothing serious is observed, even if you provide window at southwest direction. As it is, the agneya corner guest room crosses the middle point of the house, so if you keep the window in the southwest direction also, it will still be alright.
Whether you want to provide an attached bathroom with the guest room or not is entirely your choice. Vaastu shastra neither advocates it, nor asks you to avoid it.