I need some ammunition when arguing with my house builder and would be
grateful for comments.

When negotiating a deal on my new house (complete except for the
interior stuff), the sales agent agreed to a number incentives
including the addition of a velux window in the top (3rd floor) shower
room. These were listed on our reservation agreement as special
conditions with a note saying my deposit would be refunded if the
conditions could not be met. You can guess what is coming but of
course bakcing out now isn't practical.

After a couple of months the builder is saying no way citing (via
their sales person) various reasons along the lines of needing
retrospective planning permission, changing build plans and most
critically the need to erect scaffolding to install the window which
will cost thousands. But, they say, if I do it myself after I move in
it can be done entirely from the inside at little cost to me.

I am going to have to get the solicitors involved but I won't back
down - I just need some ammunition. Why would I be able to install it
from the inside but they wouldn't? Why would they need planning
permission but they wouldn't? How much would it really cost
(externally or internally fitted)? They are bull****ting but I just
need some knowledge to refute their claims.