Hi,
looking at some plans of plot of land we are in the process of buying. It has site levels from topography survey.
I am a bit unsure by the FFL on an existing property is 64.65 and proposed FFL on proposed new house is to be 66.50. the difference being 1.85. Is this 1.85 metres of difference?

Should be. Sounds quite a raise.

That was my thoughts hence the reason to double check. Also the existing property is the lower number, I would have thought it to be other way around as the levelling staff will be lifted up for the higher property if that makes sense.

Although units are not given, I suspect the levels refer to Finished Floor Level (FFL) above a given datumn, usual AOD, Above Ordnance Datumn which is 0 metres at Newlyn harbor Cornwall.

However it would be worth clarifying the unit of measure, the difference could be significant between feet or metres.

A line through the optical level is the height of collimation. If both floor levels are taken from the same collimation then the higher floor level will have the lowest staff reading as the base of the staff is on a higher level.

Yes, it sounds like the proposal is to raise the FFL 1.85m. You may want to check if there is a reason for that: Maybe it is changing position on site, to a higher part of the land, maybe it’s in a flood risk area (even then, 1.85m sounds a lot), maybe it’s proposed to partially cut a basement in underneath.

Is it a steeply sloping site?

A line through the optical level is the height of collimation. If both floor levels are taken from the same collimation then the higher floor level will have the lowest staff reading as the base of the staff is on a higher level.

But surely, on the final topo plan, all levels will be referenced back to the primary datum. The staff readings are local readings only and therefore not absolute and, as such, won’t be plotted on the final site plan.

dickie this is what I was thinking, there must be an error somewhere here as its impossible for the new house to have a lower FFL than the existing house FFL as it sits on an elevated site above existing.
Confusing!!

The FFL could be reflected to ensure the property stays dry in the event of a flood. Is the property in a flood zone (different to a flood plane)? Check the Environment Agency’s website.

When was the existing house built?

mikey74 this would look to be right as the new house is on a slope above the existing, I was thinking the lower FFL should have a higher rteading

It’ll be set to height above Newlyn Datum. So if you’re proposed is a bigger number it’ll be higher than the existing. It might be a local datum, but that suggests a very steeply sloping plot 😉
Lucky it’s not LUL grid as that is based on Newlyn -100 so that there’s no negative numbers.

aP So am I right in assuming this is 1850mm of difference?

Yes, I would say so, although it would be better to see the plans so the levels can be seen in context.

1.85m raise in FFL for the proposed development compared to the existing us quite an increase. Assuming the footprint of the proposed and existing are similar that’s a lot of material being imported. Not to t mention questions as to why the raise and how it reflects surrounding levels. Assuming that’s correct then that’s quite a lot if engineered fill and I’d be interested to understand the reasoning behind it.

FFL being Finished Floor Level, it tells you nothing about what’s under the floor. It may be made up ground, or it may just be a timber floor above a void, to cope with possible floods or sloping ground. I would get some drawings; the bare level doesn’t really tell you much.

You wouldn’t fill that height difference, you’d put the foundations in, based on the original topography and have a void below the house, with stepped access to the entrance. It would be a golden opportunity to put a basement in, or a garage sub-level.

It’ll be 1850 higher than the existing (adjacent) property.
I’m assuming that it’s a series of plots on the side of a hill?
Without any other information (quite sensible…) it’s hard to say any more. I’m sure it’s fine, if you want to PM me, I’m professionally required to have and abide by ethics 😉

150mm void, 250mm void maybe, 1.85m /1850mm difference in FFL isn’t a matter of a void that’s a fully suspended slab on stilts effectively if this a similar level to another structure.

FYI I’ve done cut and fill ops that are up to 4.0m’s of filling so anything is doable…

I’m assuming that this must be an up slope development above the existing properties. Shouldn’t be an issue provided that the due considerations are met i.e. Slope stability is considered, the geology is well known such that foundations are advanced through any potentially mobile head deposits and any retaining structures are properly founded and designed by a competent structural engineer.

Ask for a section or elevation of the proposed house. This should clear things up.

I’ve done this as well and I’d say a void is the most economical way of doing it. You wouldn’t do nearly 2m of fill for a single house as you’d still need to take the foundations down to a suitable bearing stratum, which would be below the original GF level.

As above, find out the details of the new property to confirm things.

First thing I’d ever suggest is current drainage inverts and current site access levels. However I’ve been chastised once on here today so I’m probably wrong.

I’ve done this as well and I’d say a void is the most economical way of doing it. You wouldn’t do nearly 2m of fill for a single house

housemikey74 this would look to be right as the new house is on a slope above the existing[/quote]
So it won’t be on the same spot as the existing, it will be up the slope, so a higher FFL is totally logical.

I was thinking the lower FFL should have a higher reading

You are quite right that the reading on the staff would be lower for the higher level. That’s because the higher level is, er… higher. Just scribble a cross section showing man with level and the two FFLs. It should become apparent. Bigger number for FFL = higher level, just as with contours or spot heights on a map.

That’s all a bit confusing ^.
Think of 0.000 as sea level. Then just go up from there.

The reading on the staff isn’t the level, it’s just an intermediate step in working it out, and since it depends what height the level instrument has been set up at, it’s meaningless to anyone except the surveyor.

Levels are always the height above whatever datum has been chosen – if you look on the Ordnance Survey map for the area, you’ll probably see the 60m contour somewhere down hill of the site, and the 70m above the site.