Joining New Toilet waste to existing vent stack

[cloogy]

I have built a garden office on the back of my bungalow, and want to add a loo to it. Looking at my options, I think the most robust way would be dig out a trench to the main sewer connection in my front drive, with a couple of inspection chambers as it turns two corners. (the existing loo is at the back of my house, with the drain and vent pipe running directly under a solid concrete foundation to the drain in my driveway).
However, looking at the way everything is laid out, with the existing vent pipe 18 inches from the loo, both set into the floor with 4 inch pipes, then sleeved down to 3 inches for the vent pipe, would it be legal and reasonable to use a swept tee from the new loo waste into the floor at the vent pipe, with a new vent pipe added within 1m of the new loo?
Is the addition of a 90 degree bend on the new soil pipe legal, even with rodding access?
I have measured everything and can achieve a 1 in 40 drop from the new loo to the proposed vent connection.
To be clear , this is all on a ground floor bungalow, and the new waste pipe would be above ground.
I have a sketch attached which should clarify what I'm on about.

If the answer is just no, that's fine, I'll go with the big trench, but I'd prefer not to if possible.

 

[denso13]

You'll need building regs approval for this, perhaps best to start discussions with them now and see what the council prefer.

 

[cloogy]

You'll need building regs approval for this, perhaps best to start discussions with them now and see what the council prefer.

I get that, I just wondered if it was even worth asking about this option?

 

[denso13]

I just wondered if it was even worth asking about this option?

Yes, no reason not to explore all the options.

 

[cloogy]

Thanks, I'll see what they say