Advice on negotiating with other freeholder to develop the loft?

I’ve just exchanged on my first buy to let and it’s a purpose built maisonette. I’m upstairs and there’s another flat downstairs and we share the freehold. There’s a huge loft above my flat and I estimate that for around £70k – 90k I could build a 2 bedroom flat which would be worth around £200k when finished.

Obviously I’d need to negotiate a deal with the joint freeholder downstairs and as I’m new to this I wondered if anybody has any words of wisdom on how to approach this ? He seems a nice enough chap and I get on okay with him the few times we’ve met so I’m comfortable to approach him directly about this.

Questions going through my mind are, should I approach him early on or wait until I have tenants in the property so I’m not under time pressure for a decision from him ? (as I imagine it will take ages to get planning permission etc anyway so I’m going to be in for the long haul)

Also, I’m at a total loss on what my opening offer to him should be given that he doesn’t have access to the loft anyway and we both have our own front doors and I believe I could make the loft flat with it’s own front door (i.e. no disruption to him). Having said that there’s a potential £100k plus net profit on the table and he’ll know that he is the gatekeeper to that ! Opinions on what others would offer in this scenario would be greatly appreciated.

Thanks loads
Peter