Our 1900s built 2 bed mid terrace house has only one wall in the loft separating us from next door .

Our loft has NO wall separating us from our neighbour in the other side.

The property is under offer for sale and the buyers surveyor has noted this as a Fire risk .

I have spoken to a builder who has suggested stud wall and double skin fibreboard ect

My questions are if I go ahead do I need any form of certificate or permission?

Thank you .

Fire board not fibreboard . Sorry

Very common in older terraces. A lot of them were built with the party wall stopping at ceiling level, or only every other pair got one.

On permission, I'd ring your local building control before anyone starts. A new separating wall in the loft is fire separation work, so they'll usually want a building notice and an inspection, and you then get a completion certificate. That certificate is what the buyer's solicitor will actually want to see, so it's worth the fee rather than just building it and hoping.

It also sits on the line of the party wall, so the Party Wall Act applies. Your neighbour will need to be served notice or agree in writing, and in practice the builder will need access to their loft anyway. Worth a friendly chat first, as it protects their house as much as yours.

On construction, building control will tell you what they'll accept, but typically it's either blockwork built off the existing party wall below, or a stud wall with two layers of fire rated plasterboard on each face, mineral wool in the void, and the top fire stopped tight to the underside of the roof covering with mineral wool and intumescent sealant. The fire stopping at the top and the ends is where these usually fail inspection, so make sure the builder takes it right up between the rafters.

If the sale is under time pressure, ask your solicitor whether the buyer will accept the work booked in with building control sign off to follow.