As a general rule, if the owner hired whoever drafted the construction plans, or if whoever drafted the faulty plans is an employee or agent of the owner, it is the owner who will be liable for defective plans.
How to draft a contract between two parties: A step-by-step checklist Know your parties. Agree on the terms. Set clear boundaries. Spell out the consequences. Specify how you will resolve disputes. Cover confidentiality. Check the legality of the contract. Open it up to negotiation.