The covenant would be binding so If the original developer or their successors in title found out about it they would be entitled to take enforcement action against you.
In order to remove or modify the covenant you would have to have consent from the original developer who imposed the covenant. They would probably require a fee for this. Obviously it may be that due to the age of the covenant you are not able to trace the developer or their successors in title to arrange this and so this is not a possibility.
Similarly, because of the age it may be that the developer will not find out about the breach, therefore no enforcement action would be taken and you would suffer not loss.
You can make an application to the Lands Tribunal in order to have the covenant removed or altered. This is a fairly lengthy and relatively costly procedure.
If you did build in breach of the covenant and then went to sell the property the usual way of dealing with it would be to provide a Restrictive Covenant Indemnity Policy to cover your purchaser against he loss of any enforcement action taken. These policies are generally available only if the works have been in place for 12 months and no contact has been made with the original developer.
Essentially, the covenant only becomes a problem if the original developer or successor in title (or a neighbour) finds out about the covenant and takes action
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Edited by Thomas on 2/15/2011 at 4:26 PM EST