We are currently renting a property and the back door leads to a garden that is enclosed by the house extension on three sides so solid wall and the neighbours 6 foot fence on one side so their is no exit from the  rear of the property. You would be trapped if their was a fire in the kitchen / living room and you couldn't get.to the front of the house. 

Also the front door won't open if the door to the loft extension is open and same if the other away round. Is this legal? 

Plenty of houses exist without back doors at all and only the front and are perfectly fine. The door configuration appears a poor design though. I'd check on the council planning web site to see if the extension passed Building Control.