Are there any rules that must be followed to ensure that an agreement document drawn up and signed by 2 or more parties is legal?
Yes. Contracts are legally binding agreements that are enforceable in a court of law. Not all agreements between two parties are legally binding contracts. The five key requirements for the creation of a contract are: 1. offer and acceptance (
agreement) 2. consideration 3. an intention to create legal relations 4. legal capacity 5. formalities.
An offer is plan to make a transaction, and must be made to another person and be held open until one of four things has happened: the tender is accepted, it is retracted, it is rejected, or it has expired. A counteroffer will close the original offer. Some terms in a contract must be specifically defined because they identify the basic obligations of the contract. These things can include a price, a quantity, or a description. An acceptance is when the other person acknowledges the offer and, subsequently, accepts the offer. The acceptance of the offer must comply with the terms outlined in the offer. Consideration is the bargaining process that defines the legal benefits of both parties and from one person or party to another. Consideration generally takes place in the form of monetary exchanges, property, or services.
Other requirements of a contract include competence, legality, and consent. Competence refers to the legal capacity of the parties to make a contract. This, in general, means that all parties involved in the contract are of sound mind and at least 18 years of age. Consent in a contract means that all parties involved in the contract agree to the contract's terms. It does not mean that you have to know what is said in the contract. Legality refers to the requirement that a contract must be legally enforceable.
For example, can an individual simply write up a document get another person to sign it stating that they accept it's terms and for that document to be legally binding or carry weight in court?
Yes, if the individual who is writing up a document knows how to draft a contract. But I would not imagine that doing so is normally simple.
Would there need to be any witnesses listed (as with a Will), does the document need to be notarised etc etc?
I don't think it is necessary to have witnesses, but in some certain circumstances it can help if there is any dispute about the contract.