If your friends are building a new home it is likely they will have to have HR ventilation under regulation to meet the standards and unless they build the entire building out of glass  or have some unreasonable amount of ventilation it is unlikely you will need above 25-30w/m2 under new build standards. There would be no need for mixed emitters so place UFH everywhere but don't skimp back on the loops, try to add additional loops for better control example below.

While you have the chance you can lay additional loops for the UFH for much better control allowing you to have open loop but a % of control in main rooms, this is not additional pipe laid but additional control, if the room needs 330m of UFH pipe you are just splitting it into 3No 110m loops, the UFH manufacturer will help with this. Auxiliary such as hallways and landings areas can just be open loop as they tend to be background heating or transit areas. Control loops should be away from the external walls where possible.

For example:

Living room 900w 3 loops (2loops open and 1 loop controlled by UFH room stat) Try to make the loop down the middle of the room the control loop as the heat loss will be higher along the longer edges until there is solar gain.

Kitchen 800w 2 loop (1 loop open and 1 loop controlled by UFH stat) make the loop away from the external wall the control loop 

WC 60w 1 loop (Open loop no UFH control)

Hallway 260w 1 loop (Open loop no UFH control)

Landing 160w 1 loop (Open loop no UFH control) 

Bedroom 1 320w 2 loop (1 loop open and 1 loop controlled by UFH stat)

Bedroom 2 420w 2 loop (1 loop open and 1 loop controlled by UFH stat)

Bedroom 3 220w 2 loop (1 loop open and 1 loop controlled by UFH stat)

No blending or interface control form the UFH to the heat pump just pipework straight connected to the UFH rail then on your rail connect the actuators for loops that will be controlled and the other leave off, your ASHP commissioning engineer will understand this.

Now set the ASHP to run on its own internal interactive smart stat with a compensation curve around 40@-3 and 20@20 to start or at 0.6 subject to which manufacturer you are using (this can be set correctly once you have the full heat loss, your ASHP engineer will be an expert in the manufacturers controls options) 

What the above does is covers every eventuality including solar gain over heating in the winter period, take the living room for example if the heat loss is the full calculated 900w but its a February sunny morning and the solar is offsetting 200w then the rest of the building heats on the curve and the living room UFH stat will gently close the 3rd loop reducing the capacity flow from 900 w to between 600-900 by adjusting the flow on the 3rd loop. The HP sees it DT return rise and either can divert the unused energy back into the building to other rooms or reduce its output.

The control set up above is to set the UFH control stats connected to one loop as an over heat stat, so your ASHP interactive smart stat is set for 21, this controls the whole house and the ASHP, the UFH rooms stats set for 22 in main rooms and say 17 in a bedroom and will nicely hold off that extra heat not wanted, adjust the rooms stat to 20 and the bedroom will heat up to its maximum design. (assumes the bedroom design is 20 where other rooms are 21 and 22) all of this can be designed by your fully qualified low temp heating design engineer, just make sure you find a good one.

UFH designs work perfectly well without the extra loops once balanced perfectly but people do not, I have had it where customers once agreed to a design of 20 or 21 want it 22 in the main rooms sometimes but 16 in the bedroom even though they decided 18 on the bedroom design, this system above adds a small amount of flexibility so like a deluxe design, this is my thought the second I hear someone is building from scratch.

We have been and are still experimenting with multi control and simple control, on retro fit and standard heating, open loop and steady balanced control works very well and is the go to optimal design, on new build its the same but with added single loop control makes it that little bit more control that we are all used to with stats in rooms. This method also prevents the customer setting the HP to run full power but a large proportion of loops closed, this is a big cause of low flow. 

Avoid a buffer at all costs as you can design it out on anew build.

I hope this gives you options for your friends. 

@ashp-bobba 

Thank you for a very detailed reply, the idea of having say 3 loops in a room instead of 2 makes sense if there is a smart controller fitted that can isolate part rooms. I presume you recommend loops at 150mm spacing.

I think about my own system and incorporating your examples would have added more flexibility, my controls are relatively simple, using the heat pumps WC, the problem comes with variable weather conditions. Every temperature has 2 values, take +3° on a calm dry day, the pump efficiency is at its best, the same temperature on a windy wet day the efficiency is at its worst, this is because the airflow through the pump is not as regular when its windy, the same wind draws more heat from the house, and the damp air means the pump goes into defrost more often. Because we are in a barn conversion bungalow we have a long thin design giving a large wall to total floor area ratio, the different heat requirements listed above are at least 10% and maybe more.

When my friends they get to the stage of talking to heating engineers would you be ok with me sharing your comments? i would not share your details.

Cheers Mike

The spacing needs to be designed though a software or by the UFH manufacturer to match the room design, so living room could be 200 and bathroom often 150 or smaller is poss as its usually a smaller footprint having the bath on the floor.

Yes please feel free to share the info, its just an idea thats seems to work quite well when you have the flexibility of designing from scratch.