My question is: have internal designs changed in recent years for practical and Health & Safety reasons? Could a new organ be manufactured with the same space-saving as was used in older organs?
First part of the question: As far as I can say, no, they haven’t. It is still up to the builder, consultant and buyer and their mutual relationship to determine specifics such as the use of space in any case, be it new or existing. At some point, the builder might point out that for many reasons he would rather have more space than still more ranks to fit in, and hopefully the consultant will go along with that.
As to secure access, I am not sure if there are regulations, but I suspect there are – here a builder might be able to tell more.
Second part of the question: Yes and no. Chest layout can of course be done in major or minor thirds, which considerably helps saving space and provides access to some extent without having to resort to walkboards. What usually can’t be done is space saving in the lower range: Where old builders and buyers were completely comfortable with a short or broken bottom octave (starting CDEFGA or CDE, respectively), we expect the full chromatic range throughout, with two to four more large pipes per rank to accommodate. On the chest, they easily account for up to a meter in width. To cope with that, builders in the 19th century set-off large bass pipes, or placed them in or behind the facade, or shared them between ranks (Schulze did this to a large extent, combining Gambas, Salicionals and Diapasons, Hohl Flutes and Stopped Diapasons in the bottom octaves of manual 16- and 8-foot ranks).
I often marvel at cases and the respective stoplists, wondering where it all goes. And I admire builders such as Lynn Dobson, who apparently were able to convince their customers not to demand more organ than space will accomodate. Lynn Dobson’s collection of designs, many quite original but with their own internal logic, rarely leaves one wondering how they “did it”.
The late Stephen Bicknell wrote several wonderful pieces for Piporg-L on this subject, just to give subscribers an idea of the space needed for an organ of just moderate size. One particularly concise one can be found in the list archives here. He faced the problem on several occasions, and used to be quite self-critical, relating to, among others, an organ built by our hosts which he designed. And his rant concerning St Lawrence Jewry, with its catching bottom-line, is quite legendary among older Piporg-Lers.