We have a smell of rotten eggs inside the backdoor of house. Yesterday the smell also started coming up from downstairs kitchen sink and then upstairs bathroom sink.
We tried pouring some caustic soda down the drain outside kitchen window and the smell seems to have stopped from the sinks. It abated somewhat in the backdoor but is still present.

Sounds like you have a sewer gas issue. Lack of vents, plugged vents, or even a partially blocked sewer or septic issues can cause this. Sewer gas smells can be one of the toughest things to locate. You may need to get a competent plumber in there to look the system over and start trouble shooting to locate the source of the smell. Please let us know what you find.

Welcome to The Plumbing Page at AMHD.com.
When you flush or drain do any of the fixtures bubble or gurgle?
Does the kitchen sink make a noise when the washer drains?
Can you furnish more details?
Back to you, Tom

One simple thing you can try - as soon as the odor is present, find the indoor ( or outdoor) cleanout plug and remove it, carefully take a whiff and see if the smell matches the odor you have in the house. I've been on scores of perported sewer gas service calls, and a lot of the time the odor is from another source...

Hey Mike,

I've been on scores of perported sewer gas service calls, and a lot of the time the odor is from another source... and that source would be?
Back to you, Tom

Tom:
Dead mice, nesting birds, rats, chipmunks, squirrels, snakes, bats, or even larger animals that have died in a wall cavity, attic etc.. Decaying flesh has an odor very similar to septic / sewer gas. I have traced odors to outdoor generator batteries, & batteries ( car & small equipment )in garages and sheds adjacent to, or attached to the house. Batteries can produce a similar hydrogen based odor often found in idle, energized electric water heaters.
Pool chemicals that are carelessly stored in or near living space can react with other nearby materials, and produce a rotten egg type odor. Stored fertilizer, dried bagged manure, livestock & pet food.
Mold, mildew, seasonal subterranean water, and sump pump pits. Forgotten, stored, home canned / jared food with failed seals. Abandoned dishwashers, humidifyers, freezers & refrigerators with leftover food left festering, and freezer, refrigerator, and icemaker bins & condensate trays.
Unburried pet caskets, shallow pet graves, kennels, pet carriers, pet cages, poorly maintained fish tanks, live bait tanks... I've traced alleged septic odors to all of these things during my career, and have actually found rotten eggs many times...

I forgot about these two more recent sources of offensive odors - a small natural gas leak, and a grease filled duct connected to a cooking line exhaust fan...

Verry good Mike, but along with a small natural gas leak, and a grease filled duct connected to a cooking line exhaust fan.. You also forgot sulfur water which can beat any smell you have up there for rotten eggs. So, are you saying that they should look elsewhere?
That perhaps is not a plumbing problem? Or are you just giving them other options? Regards Tom

That perhaps is not a plumbing problem? Or are you just giving them other options? Regards Tom
Doesn't the gas company add a sulphur smell so that a leak can be noticed and found?

Hey WG.

Doesn't the gas company add a sulphur smell so that a leak can be noticed and found? they add something but I can tell you this from experience. Nothing! But nothing smelled like rotten eggs like sulfur water. If you have ever smelled it you will know exactly what I mean. Cheers Tom

Mercaptan, a chemical that contains sulfur, is added to natural gas.

Yes, Tom, while decaying organic matter in a septic tank causes a " rotten egg " odor, there are plenty of other suspects, and until the source of the odor is found, all should be considered.
You & I could easily check all the obvious, accessible drain, waste, and vent connections to eliminate them as a source of the odor; it may not be that easy for the average home owner.

Sounds like you have a sewer gas issue. Lack of vents, plugged vents, or even a partially blocked sewer or septic issues can cause this. Sewer gas smells can be one of the toughest things to locate. You may need to get a compitent plumber in there to look the system over and start trouble shooting to locate the source of the smell. Please let us know what you find.

Many thanks for all of your helpful replies. Much appreciated.
The next day the smell seemed to disappear altogether. The following day however we learned that our nextdoor neighbour (semi-detached) had lifted his manhole cover and found it was completely full of water.
Our neighbour called Thames Water who are apparently in charge of the local sewers. They sent two very helpful plumbers who checked our manhole cover and that of the property on the farside of neighbour ( there is a lot of drain sharing ). Thankfully both were clear.
They then arranged for some sort of hose-pump to clear our neighbour's blockage. It seemed they were there for a couple of hours.

The plumbers confirmed that the rotten egg smell would indeed have been sewer gas and explained that it was potentially harmful. They strongly recommended that if we ever encountered it we should contact professionals like themselves to investigate rather than attempt to deal with ourselves.