Linkle, are you looking for a kit type build or an individual designed one ? Is it for you or a child ?
Try these people although might be expensive for a bespoke one. I got my dolls house from here in the 1980's and didn't think they would still be going. I think they may now live in Ormskirk but the shop is still registered in Liverpool. Lovely people, a little family business.
They were beautifully hand crafted, individual dolls houses.
Lincle, I think Greenwood has the better idea. Bespoke dolls houses are not really for children and can be very expensive , even the kit forms can be expensive. A three year old would be far better with one that is on the website shown on Greenwoods link.
I did think it might be for yourself initially.
Humankind has not woven the web of life. We are but one thread within it. Whatever we do to the web, we do to ourselves. All things are bound together. All things connect. ~Chief Seattle