Where will we go through customs/immigration? Flight into LHR then to ZRH

Just to be clear, I'm answering this based on the assumption that you already have a ticket booked (say, JFK-LHR) and you are going to separately book a LHR-ZRH (not ZUR! :) ) trip on a different ticket.

First note is that under all circumstances you will have to go through a security screening at Heathrow. It is unavoidable, even if you had been on one ticket, as Heathrow requires any connecting passenger arriving from a non-UK airport to go through security before their next flight.

You say you won't have any checked bags. The key question then becomes, will you be able to get your LHR-ZRH boarding pass beforehand on mobile or printed out, or at an airline ticket desk airside once you reach Heathrow? If so, then you should be able to avoid UK immigration and customs. The UK (unlike the US) does permit airside transit -- i.e. normally if you fly in from outside the UK and continue to another non-UK/Ireland destination, on one ticket, you can bypass UK entry formalities.

If you for some reason cannot get your LHR-ZRH boarding pass ahead of time or at an airside desk, you will have to get your boarding pass at the landside (pre-security) desks, which will require you to go through UK entry formalities to get to those desks.

Many airlines allow you to check current flight (yesterday and today) gates without a name or a ticket. It would be wise to look at your flights for a recent day and see if they flew into and out of the SAME physical Heathrow terminal. This look is done on a tab with a label like Flight Status (example from United.) I always do this for a couple of days to get an IDEA of what I'm likely to encounter, anywhere. This information can sometimes help you answer your question.

EDIT: I generally avoid Heathrow, but the last time I changed terminals I had to do immigration, because I took a general population bus to the second terminal. Note that Heathrow NOW has eGates for several nationalities and conditions, INCLUDING Americanskis. This can save 60-90 minutes!

Is there a compelling reason why you are not booking a flight to ZRH connection at LHR (or someother European hub)? And how are your returning to the States from ZUR or elsewhere. The separate flight itineary add complexities to your travel that puts you and your family at financial risk and adds transfer issues.

Note that even if your carry-on luggage complies with the airline's size policy, a full flight can resulst in some carry-on baggage being gate checked if there is not enought overhead storage to accomodate everyone's carry-on.

I too am puzzled about why you're not booking one itinerary, your US airport to Zurich. If you want to leave the airport and spend time in London, you'll need to go through immigration at LHR. If you are just changing flights in LHR to fly onward to Zurich, you'll remain inside the airport's transit zone and not go through immigration (except if the UK has Brexited from the EU by then, in which case things could change).

Either way, you are best advised to book one itinerary from your US airport to Zurich. Among other things, in the unlikely event that your transatlantic flight is massively delayed, the airline will have more of an obligation to put you on the next available flight to Zurich with no extra charge, instead of leaving you out of luck because you missed the flight you'd booked.

1) The airport code for Zürich is ZRH. I don't know where ZUR is, but it ain't Zürich.

2) Heathrow (LHR) has multiple terminals at different sides of the airport: 2+3 in the centre, 4 at the south side and 5 at the west end. You need to treat these as different airports, it can take ½ hour+ to get from one terminal to another, either "airside" or "landside". When deciding how long you need you must factor in which terminal you arrive at and depart from.

If (big if) you can get your boarding card for the Zürich flight before you leave home, than you can stay "airside" and not go through UK immigration. If you need to use the check-in desk at Heathrow, then you need to go through immigration, double back to arrivals, and check in again, followed by back out.

There will always be security. Like taxes it is unavoidable. Sensible airports do not trust incoming passengers to have been sufficiently screened at their point of departure.

Important point: If your incoming flight is late, and you miss the second flight, you have no comeback if you have two separate tickets. Makes sure you have enough time (3 hours) to allow for late flights and terminal change.

Even better: Scrap this plan and fly direct to Zürich. Or big a better airport to change at, e.g. Amsterdam Schiphol.

P.S. I haven't mentioned customs as it is a non-event, random screening only, 95%+ are no stopped.

Thanks for all the replies. We got very inexpensive flights on a flash sale round trip from the US into and out of London, hence the separate reservations from LHR to ZHR. (Saved $2,000 on airfare doing it that way and that savings is worth the extra effort to us. We’re hopeful that all will work out timing wise. )

I’ll look into what terminal we fly into, thanks for that tip.

Our return flight is from Bari Italy back to London. Separate reservation. We’re flying into and out of multiple cities on this month long trip.

My main question is how much time do I need to allow for immigration/customs & possible terminal change between our SEA-LHR flight and LHR-ZHR flight.

Thanks for the helpful info!

Go to Heathrow.com and look for Connections. They offer a helpful link for changing planes, terminals at LHR.

Sensible airports do not trust incoming passengers to have been
sufficiently screened at their point of departure.

EU has set common security standards. So transfer passengers arriving from another EEA country or a country where airport security is deemed to be equivalent to EU standard (e.g. Canada or USA) do not have go through security again when changing planes in an EU airport. But the UK has opted out of this and requires all transfer passengers arriving from non-UK airports to go through security again. One of the reasons I tend to avoid British Airways.

My main question is how much time do I need to allow for immigration/customs & possible terminal change between our SEA-LHR flight and LHR-ZHR flight.

If you have your boarding pass for your onward journey and you have carry on luggage, there will be no customs and immigration at Heathrow. You will only go through a security check. You will stay airside and follow the ”flight connections” signs to the terminal that your Zurich flight departs from.

The main thing you need to consider is how much risk you want to take in case your inbound flight is delayed, If you miss your connection, you may need to buy a new ticket. I’ve missed connections twice at Heathrow over the years due to weather or mechanical issues on my inbound flight. I also had a flight to LHR canceled when BA had a system-wide computer failure. Thus, I always plan to arrive the day before my onward flight. However, in most cases, 4 hours would probably be reasonable between flights to allow both time to connect and accommodate a flight that arrives a little late.

Do you know arrival and departure terminals? If you arrive at Terminal 2 are you departing at Terminal 2 (e.g. Swiss Air) or Terminal 5 (BA)?
https://www.airport-london-heathrow.com/terminals.php

Transfer between terminals

Please note that Terminal 2 and Terminal 3 are located within a short
walk from each other. To do so, there's a pedestrian underpass. It
doesn't take more than 10 minutes.

Heathrow Express trains and buses transfer to Terminal 4 and 5. Travel
time is about 20 minutes. There are four trains per hour. To use this
service, it is required a free inter-terminal transfer ticket, which
can be purchased from the machines in the station.

Heathrow Terminal 2 The newest terminal in Heathrow Airport, also
called the Queen's Terminal.

Most of the international services departs and arrives at Terminal 2.

Heathrow Terminal 5 Terminal 5 is a hub of British Airways.
Eventhough, Iberia also operates flights from this terminal.

It counts with three concourses; A, B and C.

I would leave a minimum of 4 hours and recognize that this still comes with risk. Have you considered taking some of that $2000 savings (good on you!) and staying at an airport hotel at Heathrow overnight? This should greatly reduce risk factor between the two flights.

When I change planes at LHR on two separate itineraries like you will have, I allow a minimum of 4 hours. You never know about any inbound transatlantic delays and I allow a big cushion for them.

My most recent flight to LHR about 2 weeks ago was delayed at JFK for 2 hours because heavy winds closed 3 of the 4 runways. We sat on the tarmac for those 2 hours waiting our turn to take off as that 1 open runway was also used for landings.

OK so not to really bogart this thread but I'm flying into LHR from the US arriving at Terminal 3 on Delta airlines on May 25th at 1:30 pm and out of Terminal 3 on BA to Marseille at 5:10 pm on 2 different tickets (YES) .....

I will go through security when I enter the terminal from my flight from the US and go back through before my departing flight to France - correct? IF we chose to check a bag because my husbands carry on is 1 inch too tall is the baggage claim then in terminal 3 as well?

I will go through security when I enter the terminal from my flight from the US and go back through before my departing flight to France - correct? IF we chose to check a bag because my husbands carry on is 1 inch too tall is the baggage claim then in terminal 3 as well?

Donna, If you stay in the transit area, you will only go through security once. If you have to retrieve a checked bag, you will enter the UK so you will go through immigration and customs on arrival and then through security on departure.

Heathrow has a connections guide on their website that maps out the steps:
https://www.heathrow.com/flight-connections

thanks Laura. I think we are going to try to just bring carry ons.

Thanks Laura. the connection guide says I need 70 minutes and I have 3 hours and 45 minutes. So if we don't check a bag we have plenty of time. I'm just worried about the carry on being too big for the BA flight :( Donna

Donna: Note that the minium connection times apply to conneting flights purchased on one ticket. The connecton guide advice notes:

Pages within this section may quote minimum connection times. These
only apply if you have booked a through-ticket with an airline or
travel agent, and any baggage is checked through to your final
destination.

I think we are going to book the later flight@ 8pm which will give us 6 1/2 hours. The other option is to spend the night @LHR and fly out at 7 am the next day.

decided to spend a couple days in London - thanks everyone