Plumbing advice for extra radiators

My heat pump is too big for my house most of the time.

I want to heat an extra room, an insulated timber office of about 20m2

The office is on the back of our garage and the garage is adjoined to the house.

I have access to flow and return pipes from radiators in the house and want to spur from those to create a loop in the office.

The extra pipe run to the new radiators would be about 8m of pipe length including rises and falls.

I am concerned about getting water that far when it is already at the furthest point from the heat pump, a Daikin 9kW Altherma 3 monobloc

Just adding a second circulation pump seems like a bad idea so I have plan to ensure flow to the office radiators.

My plan is to add a 4 pipe buffer tank in the garage fed from the radiator in that corner of the house and then add a circulation pump on the office radiator circuit fed by the buffer tank.

Would this work?

It’s quite simple to do and I think it would get heat to the office without disrupting the house heating circuits or the heat pump.