It is not compulsory, and that is not the question
You can buy a house without a lawyer, and people do it every day. The useful question is a different one: who is looking out for you in that transaction?
The notary attests and checks the legality of the deed, and informs both parties equally. They do not negotiate for you, tell you whether the price is good or review the arras you signed two months ago.
The processing agency (gestoría) handles paperwork: taxes, registration, forms. It does not review the contract.
And the estate agent is paid for closing the deal. They may be honest, and many are, but their interest is that it gets signed.
That leaves a gap, and that is what a lawyer fills: looking at the transaction from your side before anything is signed.
When it is clearly worth it
And when you can do without
Honestly: it is not always needed.
A resale home, free of charges, with a clean land registry extract, no odd works, bought from a private seller you know and without a mortgage is a simple transaction. There, a review of the nota simple and a well-drafted contract may be enough, without full support.
What we do not recommend to anyone is signing an arras contract that nobody has read. It is the cheapest part to review and the most expensive to get wrong.
Further reading: what to check before buying a home in Madrid and how much a lawyer can save you when buying.
Frequently asked questions
When should I call?
Before handing over any money. Almost everyone calls after signing the arras, and by then there is little room left.
Can I hire just the review?
Yes, and it is the most common option: reviewing the documents and the contract, without full support.
Is it enough for the notary to look at it?
The notary does their job and does it well, but is impartial by definition. They are not on your side because they cannot be.
What if I buy off-plan?
That is where it matters most, because you sign long before the home exists and there are guarantees and deadlines to review.