The planning application (DC/047839) can be found on Stockport MBC websites although there are no plans. However it would appear that Spencers did a similar scheme at Cheadle Hulme Station in which the link below has a photo and references the improvements planned at Marple. The article states about Marple -
"Work is due to start on site this winter and will take around nine months to complete. When finished, the old footbridge will have been removed and new lifts installed on both platforms, linked by a new single span footbridge.
The existing toilets will be demolished and the ticket office building will be extended to accommodate new accessible toilet facilities.
The station will have new automatic entrance doors and the area of car park adjacent to the ticket office will be resurfaced to provide designated disabled parking, with unobstructed step-free access."http://www.rail.co/2011/08/03/improvements-at-cheadle-hulme-station/
As above it looks like the car park is getting resurfaced so that quashes any rumours of a multi-storey. I'm not overly struck by the design of the new bridge/lifts, especially being on the boundary but not in the Conservation Area, however that aside I'm all for the improvements at the station. As can been seen on-site they are clearing the land and toilets a little further up from the existing bridge, so no doubt this is were its going. The existing bridge will probably remain in-situ whilst the new one is being built meaning there will be no major disturbance to commuters.
I hope that clarifies some of the issues and is of interest to people