It is ordinary good manners applied to an occasion most people attend once every few years, which is most of the reason it feels uncertain at all.
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If you are going
Reply, and reply early, because your host is guessing at food and floor space and every unanswered invitation makes that guess worse. Turn up roughly on time. Bring something small for the house, not something small for the host, since the occasion is about the building. Take the tour when it's offered, even if you can see most of it from where you are standing, because showing you round is half of why they invited you. Then read the room and go home before you have to be encouraged to.
If you are hosting
A tour is expected, so decide in advance which doors you're happy to open. Hospitality can be simple. There is no obligation to lay on a dinner, a bar or a theme, and a table of decent nibbles with people standing in a kitchen is the format most of these evenings settle into anyway. Say on the invitation if you would rather people came empty-handed, because guests find that easier than guessing.
What no official source has written down
We went looking for a British authority on this and could not find one. Debrett's, which publishes at length on entertaining at home, has no page on housewarmings at all: its entertaining guidance covers drinks parties, informal lunches and al fresco suppers, and stops there. We checked on 3 September 2026. So everything above is convention and common sense, not a rule anyone can cite at you, and you should treat anybody who states housewarming etiquette as settled law with some suspicion.
This sits in the housewarming, between when to have one and what to spend on the gift. If you want the customs, not the manners, read housewarming traditions, and housewarming gifts is what to bring when the answer is something for the house.
The card is the part people stall on, so what to write in a new home card gives messages by recipient.
Debrett's entertaining at home guidance checked 3 September 2026 and found to contain nothing on housewarmings; no other British etiquette authority publishing on the occasion was found.