Acquiring Ownership of a New House

Lets say I purchased a new house in a housing estate from a building company. To be more specific, lets say I signed the contract to purchase that house nad the site that the house is on. Before that house is fully constructed and before I move in who owns that house?

Any property belongs to the seller, until the sale is completed.
A contract is an agreement to buy the property on a date in the future. You dont own it until the sale completes.

And when does the sale complete? Is that when it is registered in the land registry office?

In the situation above, ownership doesn't change hands until the contract is completed (i..e. the deed of transfer signed, and generally the purchase money paid over in full - commonly called "closing"). At that point title changes hands.

It can happen that a vendor will allow the purchaser in to the property before the contract is completed on a caretaker basis, but it would need to be stipulated in the contract, and in that situation, title still doesn't change until the contract is completed.