Burghead Visitor Centre Upgrade

Viewing area of Moray Firth is up outside stair to roof of visitor centre. We want to make viewing room accessible from inside building which would be beneficial to disabled persons and would also be sheltered from weather.
There is no toilet at the visitor centre, the nearest being about half a mile away at the harbour. We wish to install a toilet within the new viewing area using a bio tech system to deal with waste. Standing on a scheduled site we are restricted as to what can be done but Historic Scotland are agreeable to the project.