The second spike of coronavirus cases has led to fear over house viewings, particularly in areas effected by local lockdowns. The current deadline of 31st Mar 2021 will not allow sufficient time for sellers to complete, assuming buyers are put off for many more weeks / months.
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Extending the deadline would prevent "rushed" house viewings at a time where the infection rate is high, preventing further potential spread of the virus.
The extension would also help maintain the stimulus within the housing market during a period of continued turbulence, through economic downturn and widespread redundancy (or reduction in earnings).
Those requiring a move to downsize, to reduce outgoings, would also be more able to do so.