Soil stack - under stairs toilet issue

purplebutterfly

Hi there,  I am looking for advice on whether it is possible to fit an understairs toilet on an internal wall.  The house we want to buy is a hall-to-hall design so the stairs are on the connecting wall not the the external one.  The bathroom is theoretically above the area under the stairs but a kitchen extension juts off the back of the house under the bathroom window.  
I am going back on Wednesday to try to see where the soil stack comes out but,  wherever it is,  it would still need an internal pipe connecting it to the back of the house.
Is it possible to fit a toilet on a shared internal wall and is it financially viable? Or would I be better off building a small utility room and toilet off the kitchen extension? 
I am not necessarily looking to add value to the house but have been told by friends that internal soil pipes are hugely expensive and I might need a macerator tower instead. 
Any advice is much appreciated as I am at complete loss

purplebutterfly

This is the downstairs floorplan so you can see the back of the house is further away downstairs than it is from the upstairs bathroom

Jeepers_Creepers

Can you post the upstairs plan too, showing the location of the bathroom?

As you say, the 4" soil pipe is the biggie in any layout. I suspect it'll come down to where that upstairs pipe is - can you connect into that.

Macerators are carp. 

gwynlas

I would be tempted to build a toilet/utility room at the end of thr kitchen where you already have a small window. It might be there was an outside toilet there previously depending on the age of th property.

daveyjp

You need to know where the current toilet waste pipe runs.

If it is through the front of the house at first floor level then down to the ground it will be more difficult than if it runs vertically internally then under the ground floor to outside.

purplebutterfly

Thanks everyone for your help.
We've actually decided not to make an offer on the house but it turns out it would have been fairly easy to install a toilet at the end of the kitchen where there is an existing large pantry that could be converted and extended to include a utility area.
Sadly the area is wrong for us even though the property is ideal.  
I have learned a lot about soil pipes in the last week or so and it has been a valuable education!  The house hunt continues.....