If you don't want to move,
you need to think very carefully about you proceed, You do need to get professional advice, as you only have one chance to get this right,
You need to think carefully about how much you want to invest (time and money)in this process, and if that is worth what it will achieve,
You might want to consider the option of influencing the build, you could use a mediation type process, where you and the "friend" sit down and explore how the build could have the least impact on you,
The benefits being you save a lot of time, money, stress and possible a friendship,
If you could explore this carefully with the people who plan to build you could both benefit,
If the space next to you is suitable to built on, at some point it will be, it might be wise to take the chance of influencing what is built.
If your neighbour employed my company to proceed, you would be having a build next to you, I see this situation repeatedly, and if you both can sit down together with knowledgeable guidance and work out a suitable compromise for both of you, it always works far better that way.
It's natural to resent change, but it doesn't stop it happening, If you can go down the influencing the build route, you will save yourself so much anxiety and money,
It's worth a go, if you really can't find a way to work together, then you are still free to go down the objection route.