3rd storey or rear extension?

[SpongeBob]

Hi All,

I am currently deliberarting whether to propose a 3rd storey extension to my 4 bedroom house or add another room to the upstairs rear of my property. The biggest influencer in the decision is going to be cost. the two options:

 

1. 3rd storey/loft conversion - Currently our loft is around 4ft high meaning its no way near enough to stand in. I'm assuming we would need a complete new roof and rear facing dorma style windows. with the house being relatively large, this option would easily create two rooms.

 

2. We have had a single storey rear extension around 10 years ago. This second option would be to simply build up onto the old single storey extension and simply create another 'add-on' almost. 

 

I understand that I have not included too much detail here. But with the information above, which one would work out to be the cheapest option and by how much (%)?

 

Thanks in advance

Spongy

[ProDave]

Floor plans so we can better visualise both?

 

Building on the existing extension may make rooms hard to reach or lots of long corridors.