Buying with partner who has large cash deposit

Anamox

You need a declaration (deed) of trust. You MUST be comprehensive in how you write this as you can have as many clauses as you like for one flat fee - you need to cover all eventualities in it otherwise it will become a useless piece of paper. You should also purchase as tenants in common which is likely if you're unmarried, your solicitor can clarify this for you. Cost of a DoT is around £200-300, your solicitor may be able to do it for you BUT I recommend going to a DoT website as they can advise you better with it being their speciality.

In the declaration you should state things like:

• How much she gets if you sell
• How much you get if you sell
• What happens if there isn't enough to cover the above two amounts in case your house value drops etc (like she will take 100% of sale proceeds to cover her costs, even if it isn't as much as she put in originally)
• What happens when you break up (maybe she gets full control of the sale, sale price as she put in the most OR you both must sell ASAP and both must agree on the sale price three agents give you)
• Who gets to live there if you split up and can't sell quickly (person A gets bedroom A, person B gets bedroom B OR person A moves out and person B pays 100% of house running costs whilst they remain there)
• What happens if you make profit on the house and split up (she gets 100% of what she put in, you get 100% of what you put in then person A gets 100% of the profit because they paid 100% of monthly costs or you split profits 50/50 because you shared running costs)
• Who pays for estate agent and solicitors fees in case of a sale due to breakup (50/50 or person A etc)
• If anyone else is allowed to move into the house if you split and how they'll pay rent if they do
• If you want to rent it out in case you split, can't/won't sell and both move out you can write who gets what from rent payments such as it covers 100% of the bills then 50/50 split or 10/90 to her etc.
• You CAN make allowances to change the amounts as years go on such as she may be putting in a large deposit but you may be the one who will pay the entire mortgage per month. Thus you can put for every £x you spend on the mortgage it means you're owed £x if you split, basically your "share" of the deposit will rise as you put in more. Essentially she may pay in £40k, but after 5 years you may have paid £40k in mortgage payments thus feel entitled to more money than your smaller deposit is today.

I emphasise that unless you stipulate this amount of detail your declaration isn't worth the money you've spent on it as the other person may be able to wriggle out of it using anything to avoid paying what is owed. If you end up in a disagreement once this agreement has been signed it's a case of 1) it's well written and a judge will award in favour of the reasonable person without going to court or 2) it's poorly written and you'll have to spend £thousands going to court and potentially losing your case.

getmore4less

Does not need to be that complicated for the basic financial 
the starting point is total cost to buy and the end point is net proceeds after costs before paying of any debts.

With persons P1 and P2 cash C1 & C2 and a mortgage paid in proportions M1 and M2 they can be describes as a % of the starting costs and % of the mortgage
P1 owns (C1+M1)/(C1 + C2 + M1 + M2)   owes  M1/(M1+M2) of the debt
P2 owns (C2+M2)/(C1 + C2 + M1 + M2)  owes  M2/(M1+M2) of the debt

At any point in time they each own that share less their share of the debt. 
that works for all values of cash in, house value(up and down) and mortgage,  the numbers can go negative    
A slightly harder one is the can't pay won't pay when it comes to the mortgage, for that you get 2 basic choices
1.  adjust the debt share if it is likely to be temp situation with one not paying.
2. do a imaginary sale/purchase and calculate new ownership.

ONLY costs of ownership should be included as part of the ownership calculation, NOT occupation costs 
(simple guide, pretend you were renting, ownership cost are those of the landlord, Occupation those of the tenant)

Agree on the other stuff there needs to be a good list of sale triggers, buy out options  occupancy rights to cover the what ifs this happens. 

Thus you can put for every £x you spend on the mortgage it means you're owed £x if you split, basically your "share" of the deposit will rise as you put in more. Essentially she may pay in £40k, but after 5 years you may have paid £40k in mortgage payments thus feel entitled to more money than your smaller deposit is today.

This is wrong, the cost of renting the money(interest) to buy a share should not be include in the calculations

simple example  £200k house P1=£100k cash  P2=£100k mortgage   they obviously own 50:50 with P2 owing the debt.

25years at 3% P2 pays  out £142k on the mortgage do they now own more than 50%