question: drains, ventiliation, air admittance valves

I am in the process of renovating my Victorian house. It has a downstairs bathroom, and at present the toilet connects to the main drain via a pipe in the floor. The bathroom sink and bath connect to the drain through a drain gully in the yard. The kitchen sink connects to the drain through another drain gully in the yard. I think these gulleys are known as bottle gulleys, as they have additional water traps in.
At present the drain is not ventilated at the back as there is no soil stack / ventilation pipe, nor an air admittance valve. Looking down my street, it looks like the houses originally ventilated through a vent at the front of the house, just above ground level in the (small) front garden. When these gardens were resurfaced, presumably this vent was removed. I recently had a CCTV survey conducted on the drains, and the company suggested that as the drain feeds into a siphon trap at the front of the house, and the front downpipe from the gutters feeds in above this trap, that this provides ventiliation in place of the original vent.
I want to replace the drain gullys in the yard and resurface the yard (I'm already lowered the ground level in the yard to solve a rising damp problem). However I don't want to complete this work, only to find later on that the drain ventilation is not sufficient. Looking at my neighbours house, they have had this work done, and when the builders did this, they installed a ventilation stack connected to the drains in the yard.
I asked the builder (he's specializes in landscaping for gardens) I intend to use to replace the gulleys / resurface the yard if I need to do the same, and after consulting a friend, he suggested I would be better with an air admittance valve.
However I think the valve will look rather ugly in my bathroom, particularly as after reading the building regs I'm not sure if it needs to be higher than the waste on the sink. I guess it can be boxed in, but this isn't ideal either ...
So my questions are:
1. Do I need to ventilate the drain, or is the current arrangement with the downpipe sufficient?
2. If I do need to ventilate the drain, should I ventilate it using a stack like my neighbours?
3. Alternatively am I better using an air admittance valve?
4. If I am better using an air admittance valve, am I correct in thinking it needs to be 10 cm higher than the sink waste? Or, because the sink and bath connect to the drain via drain gulleys which have their own water traps is this unnecessary and can I put the valve at the same height as the connector between the toilet and the soil pipe going into the floor?
Any advice here is greatly appreciated, thanks a lot
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On 8 Apr 2004 02:49:49 -0700, a particular chimpanzee named (Mark Butler) randomly hit the keyboard and produced:

A brief history of drains:
In Victorian times, most of the appliances in the house were not trapped; the only trap being an interceptor at the connection to the sewer. The sewers were vented either with those small vents you describe, or larger cast iron stacks at the end of a long run.
With the advent of traps to all appliances, the interceptor became redundant, and was often replaced if the house's drainage system was renewed. A vent pipe was required therefore to vent the branch drain and to prevent siphonage of the traps. These vent pipes should be at least 900mm above any opening window, otherwise the smells will waft into the house. An untrapped rainwater pipe connected to the drain is acting as such a vent, and should be altered to prevent this (either put it into a trapped gully or remove it).
Air-admittance (aka 'Durgo') valves are a recent innovation to allow air in to prevent siphonage, but not allowing the foul smells out. They can be used where there is at least one open stack per five houses on a sewer. The valves should be fitted above the flood level of the highest appliance attached to them, which in your case would be the top of the toilet pan. An unvented stub-stack can be used to a ground floor WC if the branch is less than 6m from the vented drain.
Without knowing the distances involved, it's difficult to give proper advice. Have a word with your Council's Building Control section.
Hugo Nebula
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