The couple is going to swap their two-bedroom cottage at Kensington Palace for a Grade II-listed 10-bedroom property near Windsor Castle.
It has been reported that there has been tension between Harry and William recently with the pair living close to one another.
However, The Sunday Mirror newspaper is reporting that they simply want a larger space so that Doria, 62, can have her own quarters.
She wants to be a 'hands-on' grandmother to help them once the baby is born.
The pair will move from Nottingham Cottage in the grounds of Kensington Palace to Frogmore Cottage half a mile south of Windsor Castle.
A source said: "With a baby on the way, there wasn't the option of the Duke and Duchess staying at the palace as Nottingham Cottage is tiny and none of the bigger apartments is available.
"Doria will be staying with them for quite long periods and they need a larger place."
Their new home is a gift from the Queen for Harry, 34, and Meghan, 37, as they prepare for a new life on their own.
The royal source added: "Frogmore Cottage isn't massive but it's certainly big enough for Doria to have a separate area there.
"The couple are bound to have a pad somewhere in Kensington Palace but Frogmore Cottage will be their main base.
"It's not too far from London in terms of getting to royal engagements. Their office and staff will stay at Kensington."
William and his wife Kate live in a 20-bedroom apartment at Kensington Palace.
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