Is it renovation or rennovation?

It's renovation, with a single n. Rennovation, with a double n, is a misspelling, though a common one. The word comes from the Latin renovare, to make new again, so there's no reason for the n to double. People trip on it because plenty of English words do double their consonants (announce, connection), and because the double n version turns up online often enough to start looking plausible.

The spelling matters more than you'd think in building work. If a quote or contract says 'rennovation works', the document still stands, but it tells you something about the care that went into it. We put real effort into the wording of our quotes because that piece of paper is what protects both sides once a project is under way. A builder who is careless with the paperwork before the job starts rarely gets more careful once it has.

It matters in searches too. Google usually corrects rennovation for you, but smaller directories and trade sites often don't, so searching with the wrong spelling can hide decent local firms and surface pages nobody has proofread. If you're comparing builders for a renovation in South East London, spell it with one n and you'll see the full picture.