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Thanks for all the very helpful and interesting responses. I said the access and aesthetics reasons are spurious because - I checked with Vaillant about access for maintenance, and in our case there is no problem because there will be enough space around it, and we will have ladder-free access to the roof in question, and a safety rail at the edges. As for aesthetics, they are of course mostly subjective, but it certainly will be much less noticeable (to us) on the roof than at ground level, because we wont go onto the roof very often, but we use the garden a lot. And we wont have much space for hiding it in our relatively narrow 7m garden. (Its a typical Victorian terraced inner city house, not a detatched suburban one like in Nod's photo.) The architect and engineer both know or should know all these details - thats why I dont really understanding their strong positions on those two points. Admittedly a roof position is more noticeable to neighbours. (The one in Nods photo is huge - probably double the size that we will have. And while not beautiful, its not really offensive. I would not call it an eyesore!) I think the issue for neighbours is mostly about noise, but if they hate the noise, and can easily see it, they will inevitably hate the sight of it too. Hmm. That is something I had not considered before.

Hi all, We're planning to retrofit our end-of-terrace Victorian house. We would like to ask for advice about the location of the ASHP outdoor unit. I want to keep the garden for plants if possible. So, I want to place it on the flat roof (1st floor level) of our new extension at the rear, and position it to face the neighbours (Alternatively, it could be positioned 90 degrees to face the rear garden). However the ASHP engineer has argued very strongly against a roof location and wants us to put it in the garden, facing across it, and our architect very much wants us to follow his advice. They have explained their reasons, but I don't find them entirely convincing. The reasons given and my sceptical reactions: 1. Neighbours: they say it will annoy/infuriate the neighbours. The next door property is divided into a ground floor flat, and a separate maisonette upstairs. The boundary is about 3.5 m away. So their wall would be 7m away (see the image attached). I think this is far enough away that they won't feel the cold air and will barely hear the noise (We went to visit a similar unit in a local property and it was surprisingly quiet. Just a low hum.) The neighbours on the ground floor are friendly and I think probably won't mind it at all, because it will blow above their property. The neighbours upstairs are on a shared short term rental. I assume that it's usually home-owners, not so much short term renters, who complain about neighbours' heat-pumps. In any case I will consult them in advance and take their reactions into account. 2. Noise & Vibration : We will hear some low-pitch noise (like humming) when the unit is in operation, if nearby windows are open. Also, they say that even if they are closed, the vibration could be transmitted into the building. The nearby windows are shower/bathroom - there are 2 bedrooms that have windows which could be affected (see the image attached). I think this shouldn't be an issue if the unit is carefully positioned - EG mounted on a vibration-absorbing block and we could even mount the whole thing on a concrete block which would absorb the low frequencies, but under that it still would be supported by the timber roof structure. Perhaps it could instead be fixed to the wall which is solid 9 inch brickwork with external insulation. 3. Servicing: Our engineer said Vaillant is said to be the best choice and they also have a good service network. However, he said their maintenance engineers have strict safety rules, and they will be unwilling to attend a roof installation. Work by another company might invalidate our guarantee. Furthermore current ASHP technology uses a Propane (R290) refrigerant which has some additional safety requirements - and he said this will also present problems for a roof location. However I have checked this issue directly with Vaillant who said its ok if we have: the required space around the unit - generally 1m to one side, 500mm to the other (yes, there's enough space). access to the unit not requiring use of moveable ladders (yes, we will have ladder-free access through the shower room windows). safety rail around the flat roof (yes, we can provide that). I believe that we will also be ok for propane regulations as we will have no windows or open drains within about a meter of the unit. 4 Aesthetics. The architect is strongly of the view that a roof location will spoil the view from the garden of the rear elevation. I find this rather odd - I think it will be barely noticeable, especially edge-on, but an ASHP in the garden will be certainly much more noticeable, front or rear. So I think the Aesthetics and Servicing objections both look spurious. I guess 1st & 2nd points are the ones I really need to consider. What are your thoughts? It seems to me that if we are careful, the risks of annoying noise and vibration are small, but they cannot be ruled out. On the other hand we would keep our garden space. So it is not clear which is best, and I would expect the engineer and architect to say this. They do admit that it is ultimately up to us, but every time they say it, they also keep repeating that their professional advice is that a roof location would be a very big mistake. I am puzzled why they have such strong opinions, and feel the need to resort to spurious arguments. What is your advise, opinion and experience ?