Planned start of construction of the house meaning

Why? The contract does not include penalties for the builders nor for the BU

You signed a contract with the general contractor. I give you money, you give me a house, delivery time not specified. Now you want to get out of the contract. Do you really think that is possible without costs? You should expect about 5-10% of the agreed construction sum.
But it cannot be that a contractor signs a contract and then no house is built. No matter when, you cannot treat people like that, that only ruins people. There must be an alternative other than just losing.

what do you mean a house is not being built? That is not even up for debate, only WHEN it will be built is unclear... and there were various documentaries showing that if a contract is formulated too vaguely, you rather have disadvantages. Who checked this contract?

A lawyer sent a letter to BU, where we as clients ask that the foundation slab be completed by ...... to have such a date for the start of construction, but until now BU has not reported.

A lawyer, he sent a letter to the BU, where we as clients ask that the foundation slab be finished by ...... So that there is such a date for the start of construction, but so far BU has reported nothing.