My neighbours want to dig out a raised bed and then dig out their lawn to level it with their existing patio. This will leave my garden higher than their garden, from a minimum difference of about a foot, to a maximum difference that looks like it’ll be more like two or three feet. They’re proposing to replace the existing wooden fence between us with concrete posts, then concrete panels at the bottom to support my garden, and a new wooden fence on the top. Ultimately, that won’t make much difference to what my garden looks like so I don’t think I mind.

But, I wonder if concrete fence panels are meant to retain soil? Bearing in mind the whole of my garden is on a slope that is lowest where it meets theirs, that’s a lot of hill behind it to push towards a concrete fence panel that’s basically decorative? Should it be a retaining wall? Or is the height difference not enough to warrant that? Is a garden fence a party wall thingy that needs something formal doing with it? I don’t mind them doing the work, I just don’t want to be faced with my garden sliding down the hill, or their work affecting the wall that runs along the back of the houses and separates us from the road – again, since it’s all on a slope and it’s an old wall, I imagine that it wouldn’t take much change in pressure down their end to create cracks up hill at mine.

Should I be asking for anything in writing? I made some noises suggesting they needed to get it properly assessed rather than just starting digging when they asked me about it. I don’t want to be difficult, I just want to know it’s been planned properly and I’m protected if things go downhill (literally and metaphorically).

with gravel boards (concrete panel between concrete fence posts) you can go to about 900mm max for short sections. 600mm is a more normal maximum. if over 600mm i`d ask for structural calcs to show the wall wont fail over time.

the amount of stable retained height will vary with teh size of teh embedded post and its foundation and potential load on the top. they might need more posts or a more formal kingpost wall arrangement

Atleast if it fails your garden gets bigger.

I’d insist on drainage aswell depending on the soil. You don’t want a boggy bit so they can play landscape Larry next door.

Actually nah. I’d insist there was a buffer 1m from the fenceline and they can slope down to a shorter wall.

shouldnt need drainage as the gravel boards let water through. if you are on teh high side you wont see the concrete boards – just the timber fence over (well maybe a bit of concrete but not much.

its a really common construction detail where a full brickwork wall isnt needed for small retaining on boundaries to level gardens.

Is it just lawn? If so then it sounds OK. Wouldn’t be happy if there were any trees or large shrubs as the roots may crack the boards. It may limit future planting plans a bit. I’d try and minimise the hassle and keep on friendly terms as much as possible. If it is far from the house then any future issues will be worse for them than you.

the amount of stable retained height will vary with teh size of teh embedded post and its foundation and potential load on the top. they might need more posts or a more formal kingpost wall arrangement

this is the key point tha is often overlooked.

the concrete boards will (probably) be strong enough for the loads suggested (dont quote me on that).

but you are relying on the embedment of the post and or the weight of the concrete bedding that post in to stop the whole thing tipping over. And faced with the prospect of a lot of extra digging many people will scrimp on this bit, because it will all be hidden.

with soils just becasue it works for a day, doesnt mean it will work forever.

The concrete panels will be fairly thin, reinforced concrete. That may do the job just fine, but the panels will be in contact with your soil, so damp most of the time which could lead to corrosion and ultimately failure of the reinforcement. That might be far enough into the future not to worry you though. If I was in your position and wanting to be as cooperative as possible I would ask your neighbour to get an engineer to assess it and (hopefully) provide a letter of comfort for your reassurance.

It’s likely that the Party Wall Act will apply, which means there are formal processes they should follow to notify you. It applies to garden walls, including retaining walls, but not fences, so my guess is that it does apply to making a new retaining wall, but you might want to check with a Party Wall Surveyor. On the other hand, it may not be worth the trouble of insisting on the legal process if you can resolve things otherwise.

I have a couple of small sections of retaining wall using gravel boards, one where I’m the high side and one the low side. The high side is about 40cm high, tapering to zero over about 6m, and the pressure of the soil is causing the fence posts to lean a bit. I’ll either need to replace them and dig them in deeper [how deep depends on the soil] or build a proper retaining wall at some point.

Where I’m the low side there’s about 50cm drop, but only for 1.5m, as there are raised beds on my side that support it; there’s been no movement there.

My gravel boards are about 25 years old and are starting to break up due to corrosion of the reinforcing steel. That’s a point in favour of a proper retaining wall with a good thickness of concrete over the reinforcement; in the medium to long term, your land is at risk if the wall isn’t looked after. The concrete is alkaline and prevents corrosion of the steel, but over time the CO2 in the air causes the surface layer to lose the alkalinity, so the steel rusts. The more concrete there is covering the steel, the longer that takes.

The length of ground behind the wall won’t make any difference unless it’s steeply sloped [over 10º, say] or badly drained. They do need to make sure there is drainage behind the wall; although water will escape between the panels you don’t want a mass of wet soil pushing on them. Which is a possible problem, as that’s your soil, and they ought to dig it out and put gravel in for say 0.3m behind the wall. The top layer could go back to soil. If you don’t like the idea of them doing that, for which they definitely need your permission, they should build the retaining wall 0.3m inside their own land (with gravel boars if they wish) and keep the boundary on level ground.

Hmm… I hadn’t thought of the drainage issue, it’s likely to get pretty damp on my side, but I guess the worst bit will be where I built a small pond anyway (because it’s so damp!)

I don’t really get how they’re going to dig down far enough to put posts in that will both hold the concrete panels in place against the soil and support a 6ft wooden fence on top of that. Those are some long posts! And dig holes deep enough without standing in my garden or having mine collapse into theirs. Maybe they’ll realise it’s not the DIY job they thought and give up.

Thanks as ever for the advice, hive mind!

I was going to mention the height of the fence…would it be to your benefit if they had 2ft of concrete boards and a 4ft fence…you get a better view and they still can’t see into your garden?

Party wall stuff may cover it but if its currently ‘your’ boundary to maintain, I’d want some advice to ensure any future headaches with replacing a storm damaged fence panel is now going to be their problem and not yours! I’m in an exposed location and have had concrete repair spurs crack straight through in storm conditions so concrete posts are not bullet proof.

I can see I’m going to have to look at my deeds. I think I had to put in the fence that’s there because it was mine to maintain – I don’t want a liability when I sell the place. I don’t really want it to be any higher than it is right now, and I don’t want to be looking at a wall of concrete either. Argh! All this for a summer house?

I’m not sure if it’s been mentioned by the installation of the wall will be hardest bit.  Their proposal may work, a normal cantilevered wall has twice as much structure embedded than retaining – so if 1.0m retaining the posts would theoretically be 3.0m long, pretty sure most garden structures that you’re talking about won’t be anything like this!  Anyway this still leaves the installation of the concrete panels, I guess they would be installed with top down construction ie place the first panel at your garden level, then reduce their side and lower the panel as they excavate and install more panels.  If your garden is on sand, this won’t be easy.

As others have said – the Party Wall act will come into play.

Personally if in the final condition your garden won’t be affected – or returned to how it is now I’d let the neighbours do this,  but it’s not a small undertaking by the sounds of things.

Edit: I’ll be overthinking this as an engineer 🙂

The Party Wall etc Act would only apply if there is, or will be, a wall on the boundary line. Or if they will be excavating below the bottom of the foundations of your building(s) and within 3m of them. Under common law they cannot remove the support to your property.

Having said that I would try to keep it simple as I said above. Ask them to involve an engineer and take it from there…..

I don’t want a liability when I sell the place.

Quite right. And you shouldn’t have to.

I don’t really want it to be any higher than it is right now, and I don’t want to be looking at a wall of concrete either. Argh! All this for a summer house?

No reason why the fence should be any different, and the concrete wall, if needed, will all be below ground level on your side.

But it’s not your problem, it’s your neighbours’. Explain your concerns to them. They have a choice:
(a) they work out a way to build what they want to, that you’re happy with, and doesn’t leave you with a potential liability. You’re under no obligation to agree anything.
(b) they build the retaining wall on their side, far enough away from the fence that it doesn’t affect your land.

The Party Wall etc Act would only apply if there is, or will be, a wall on the boundary line.

Agreed – but a retaining wall is a wall, I think, even if it’s made of concrete panels?

Yep a retaining wall is a wall…

Why not get them to batter down so there is no need to a retaining wall

I’d insist they put a low retaining wall a foot or so on their side of the boundary, solves a lot of problems, especially if it’s your boundary. What they are suggesting is a bodge. My selfish neighbours did the opposite and built up behind a similar concrete fence. I’m expecting one day to find their patio in my garden.

If it collapses it’s their problem but thats not much help if it takes part of your garden with it.