The Yorkshire Wildlife Trust – which looks after the bog – fear the development will damage the ancient site.
A government planning inspector will hear evidence from York Council, the wildlife trust and developers Barwood.
The decision on whether the development can go ahead – despite being rejected by the council’s planning committee – will be made by the Secretary of State and a date has not yet been set for when it will be announced.
The inquiry is set to take 12 days at the Citadel on Gillygate – but planning inspector Paul Clark warned it could take longer.
The proposed development site, off Moor Lane and north of the bog, lies within the Green Belt and Mr Clark said the key issues to be discussed at the inquiry will be whether there are special circumstances to allow this land to be used for homes – as well as the impact on the bog, on roads in the area and on meeting the need for more houses in the city.
David Manley, speaking for the developer at the opening session, said 35 per cent of the homes will be affordable, the scheme will include new public open space and there will be a 124 metre-wide protection barrier created between the development and the bog.
He said: “Very special circumstances exist to justify the proposal. York is in the midst of a housing crisis. York accept that in order to meet it’s housing needs now, development will have to occur on a significant scale.”
He added that a key issue is the impact on Askham Bog – but that there are complex arguments over this which will be outlined during expert evidence.
Stephen Morgan, representing the council, told the inspector that Askham Bog is a site “to be cherished and protected”.
He said: “As a site of special scientific interest in itself, the bog achieves significant legislative and policy protection. The proposals would cause significant harm to the Green Belt.
“There would be very significant harm arising from these proposals.”
He added that the council disagrees with Barwood’s claims about the impact of the development on the bog’s hydrology.
Emma Fenelon, representing Yorkshire Wildlife Trust, urged the planning inspector to keep the word “irreplaceable” in mind – saying the bog means a lot to residents.
She said: “So much of the natural world is in peril and now, more than ever before, ancient places like Askham Bog must be protected. These are Sir David Attenborough’s words.
“In the midst of such technical arguments one might be forgiven for forgetting that Askham Bog is an ancient place. It is already almost surrounded. Proposals to mitigate the effects [of the development] are unworkable.”
Coun Anna Perrett, Labour candidate for York Outer, and Coun Stephen Fenton and former councillor Ann Reid will also speak at the inquiry.