No it most certainly not normal to bury vents!
I don't know what is worse, the fact that your air bricks are buried below finished ground level or that your house builder has put it in writing to you that this is perfectly acceptable!
Could you send me an
e mail contact with a copy of their letter attached? I will not publish your name and address.
Whilst the plastic part of the air brick that you can see is rectangular, they are normally fitted in a periscope ducting so that air from above ground can be channelled to ventilate the ground floor suspended floor void, which can be below the finished ground level.
Provision of air bricks at 2 metre centres is an NHBC Standards requirement (5.2 - S3).
Surely it must have occurred to someone that air bricks can not ventilate any air if they are buried? Even worse, they are more likely to enable rainwater to get into your home under the floor possibly causing damp inside your home in the worst case. The situation you have now is that the excavated air bricks are at the bottom of a "hopper" meaning rainwater will drain off the ground down under your floor.
This really must be addressed.
If as seems likely the builder is not going to do anything,
write to the NHBC or your warranty provider.
This should be a concern to them especially as the home is timber frame construction as it is even more important that the cavities are also ventilated to let the frame dry out and help stop it rotting.
There are too possible reasons:
1) The finished ground level is too high.
2) The air bricks have been put in too low. This could also mean that your damp proof course (DPC) is not (at least) 150mm above finished ground level as it is required to be, which is also serious, more so with timber frame!
It would appear the only solution would be to excavate the ground levels adjoining your house and re lay paving to the correct levels. Your landscaping would probably also need to be re modelled. Note that if ground levels adjacent to your house are lower than the surrounding ground levels, you will need also need some form of drainage (gulley or channel) to prevent the paths flooding.
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